I get headaches in the car, too, especially on long rides when it is sunny. I link this, personally, to the fact that I am very prone to motion sickness, and the headache is one of the symptoms. If I don't take anything, I will get nauseous. I often choose to be the driver because I don't get sick while I am driving - just as the passenger, especially in stop and go traffic. I have always gotten a headache at a particular point in my monthly cycle - I think that's pretty common. Neither scenario for your daughter stands out as anything I would worry about. Honestly, she's 15, so I would just tell here where to find the Tylenol or Advil and call it a day.
My 10 year old son gets migraines. He is on a daily medication (Elavil) to try to prevent or decrease the frequency, and he knows where the Excedrin is. He knows that if he starts feeling a headache coming on or his vision starts to blur or become hazy (he gets an aura a lot of times before the migraine starts), to take an Excedrin with a little can of Coke, and to lie down for about 20 minutes in a dark quiet room. If he doesn't, the migraine will progress until he is throwing up and in a lot of pain. The school nurse has the same protocol at school. He just spent 1/2 the day in the nurses office yesterday doing this. He is under the care of a Pediatric Neurologist and has had EEG's and MRI's done to rule out anything physical, both of which, thankfully, came out fine. He has probably had migraines his whole life, but he's only been able to identify them to us since about age 5 or so. Looking back at the symptoms and his behavior changes before/during a migraine (he gets really crabby), I'm pretty sure he was having them as a toddler, too. He's generally a very easy-going, sweet kid, but before we knew about the migraines, there were times when his whole demeanor would change and he would just throw up out of the blue. We would put him to bed thinking he was coming down with something, but he would sleep for a few hours or overnight and when he woke up, he would be perfectly fine. At the time, we thought he must have just had a 24 hour bug or ate something that didn't agree with him...it wasn't until he was about 7 or so that we started to realize that the throwing up came with a headache.