I figured it was more severe than anything I have. That's why I made sure to write, don't just go on what worked for someone on the internet, and chuck what your doctor has you do.
Their dr. is currently a pediatrician? Perhaps he's not up on the latest info as that is not his specialty. If you ask a GYN who specializes in period stuff, they will tell you yes, it
may be a cause of anemia. It depends on how heavy the flow, how long, other variables, etc.
Yes, she has a condition known as Pica. Google it. I found out, on my own that my previously diagnoses "slight anemia" had gone to severe, because I was majorly addicted to Altoid mints. I was CHEWING Altoids. Who
chews Altoids? One is so strong.

But, not for me. I was chewing two at a time, and could go through a whole tin in an day & a half. I've read other people went through a couple tins a day, until they were diagnosed.
People who have Pica chew what are considered non-nutritious food items, like ice and mints, to totally non-food items, like chalk or toothpaste, because their body is craving iron to such a degree, it's trying to chew & suck out iron from whatever it thinks it can get iron from, even if the product - ice, mints, chalk, toothpaste, does not really contain
any iron.
I'll see if I can dig up the runners forum thread where many people stated they had pica and other severe iron-deficiency anemia symptoms, so you can check & see how
many other symptoms your MIL probably has. If she has Pica, then she most likely exhibits many of the other symptoms, too. When I read that thread, I found I had ELEVEN of the symptoms.

I almost ran out to the 24 hr drugstore at 3am when I read that thread, to go get iron supplements.
Instead, I went to my health food store the next day. I was recommended the Hema-Plex I mentioned above. It has 85mg of iron. Is a vegetarian supplement.
Amazon has some reviews for it. Perhaps she will
try it as you can get it over the counter at a health food store? (This does not mean she shouldn't
also get medical testing.)
Within several
hours, a lot of the brain fog & exhaustion I had lifted. (The rest is due to the hypothyroidism, but that's another story.) And in a day and a half, my cravings for Altoids was totally gone.

That was a year & a half ago. I still have 3 tins of Altoids I have never touched.
Of course, then I finally went and got proper blood testing to find out what my levels are.
ETA: I found the runners thread. It really helped me, as instead of a list of symptom, reading about people writing about anemia through their layman's terms
experiences helped me realize how bad my anemia had gotten. The usual list of anemia symptoms are the same as the symptoms for so many other issues. Reading about the chewing of ice or mints, planning so I could sleep all day, helped nail it for me. This may help your MIL to go get checked.
http://www.coolrunning.com/forums/Forum4/HTML/010015-2.shtml