(You've been warned, this is going to be long!)
Op, I'd like to share 2 things with you, hopefully they'll help you or anyone else reading this thread with the same fears.
When I was in high school I had some health problems. I started throwing up every now and then legitimately. However, I started to realize that I was losing weight. I felt so out of control during this time in my life and I'm a control freak when it comes to my own life. I was sick a lot and I felt like my body had betrayed me. Then as I noticed I was losing weight I started making myself throw up. The weight kept coming off and I felt great, I finally had control of my body back! I wouldn't binge and purge, I'd just eat and then go throw up. I started eating less and less as well. Did my personality change? Of course. It wasn't drastic, but deep down (unconsciously) you know it's not right or normal to be throwing up everything you've been eating and chronically doing something wrong does wear on you.
No one at school noticed a difference in me other than some lost weight, or at least they didn't say anything (teenagers are very self-obsessed and are therefore not the most observant). I was a great student, straight A's in all AP classes (someone you'd think is smart enough to not be doing this to herself), I was in sports, orchestra, debate, an officer in a few school clubs, etc. Eating disorders don't discriminate and they don't play off the logical part of the brain, they can affect anyone, anywhere, in any walk of life.
I went from a size 12 to a size 2. It eventually got to the point to where you could count all my ribs from the front and back. Eventually my mom caught it (I think she was in denial for awhile, she was always on me about my weight and wanted me to be thinner, she was so ecstatic when I got to a size 2 and started bragging to her friends about it

). She confronted me about it and made me eat dinner and breakfast at the table every day and made me sit there and do homework or whatever until it was so long after I'd eaten that I wouldn't be able to throw up (throwing up has always been extremely painful for me so I could only do it if it was right after eating and my mom knew this). For me this is all it took.
It was hard to stop and I hated my mother for taking away the one bit of control I felt I had in my life. I saw her as just trying to be controlling herself (she is quite the control freak). However, if she hadn't gotten me to quit I probably never would have. After that I graduated and started dating the guy I'd eventually marry and I was smart enough to be upfront with him about it so he was always on the lookout for signs (I still didn't feel 100% confident in my ability to stop yet). He constantly cooked for me and kept an eye on me after eating. Now I'm heavier than I've ever been (definitely overweight) and yes I wish I was at a healthier weight, but I feel more beautiful than ever, and that comes from the inside (why couldn't I get to this stage of my life when I was a teenager and slim I'll never know, I'm too young to say it, but youth is wasted on the young

).
So, based on my experience I'd recommend confronting your daughter. You might try talking to your doctor or to a support group to get advice. One thing is certain though, if she does have an eating disorder she's not just going to wake up one day and decide to quit, and even if she does I wouldn't risk it because it could be in hospital bed that she eventually comes to that realization.
It's also important to note that eating disorders have long term effects on health, and the longer you have the disorder the worse the effects. Women who have had bulimia, no matter how long ago, are at significantly higher risk for birth defects in their children. That shocked me! Anytime I start to stress about weight in an unhealthy way, I remind myself of that, throwing up doesn't just hurt me, it hurts my future baby. I'm sure there are similar stats for anorexia. I also believe that it's like being an alcoholic, it's not something you completely cure, it's something you always have to be mindful of.
Now the second thing. This could be a lot of things, depression, eating disorder, or just a health issue. My little brother started eating a lot less and lost a lot of weight. After a lot of tests they've determined that the (excuse my lack of medical knowledge here) flap that closes off the stomach from the esophagus doesn't shut so whenever he eats his stomach forces the food back up.
The best thing to do is get her to a doctor. Let her know that you're concerned and that she's going to be getting help until she's at a healthy weight again and maintains it. If she really is fine like she says it won't take long for her to be back to normal and maintain it.
It's important to know that your daughter may have an eating disorder and she really believes in her conscious mind that she doesn't. I didn't believe it in myself for a long time. When she tells you she's fine, she might truly feel that way.
Anyway, something's wrong and it's best to get an accurate assessment and diagnosis. If it were me I'd talk to her about it before taking her to the doctor. If you bring it up with her at the doctor's for the first time, it would feel to me like airing dirty laundry in public. Let her get a grasp on the fact that you think something is wrong first before bombarding her at the doctor's, that will embarrass and overwhelm her and in my opinion make things worse.
This is all just my opinion based on experience I'm not an expert and don't pretend to be. Getting professional help is best and if it is nothing, just the paranoid worryings of a concerned parent, I don't know a professional in the world who wouldn't think you did the right thing by coming to them anyway, they won't think you're wasting their time

. This is serious stuff and your DD is so lucky she has a mother that pays that close attention to her.
Here's a

for you and a

for you and your daughter. Good luck and please keep us updated. It hurts me to think of any girls going through this.