I'm not a girl so I've never had a first hand period experience, this is what I learned in health class. If you're body fat is too low, your hormones could be a bit imbalanced. In fact, Olympic female runners often have had to gain body fat in order to get pregnant. Running almost every day seems like a lot, I'm 16 and I haven't ran competitively for 3 years and I can still run a 5:49 mile while training once every 2 weeks (although I'm doing other sports and I used to run about 5 days a week).

Yes! Especially if you are limiting calories or not eating enough to sustain your level of activity. I lost my periods for over a year due to overexercise (running & exercising 4 hours a day) and not taking in enough calories for that level of activity. You want to be careful with it. Going without a period for too long can lead to osteoporosis and future fertility problems. My cycles regulated once I curbed exercise and gained weight into the normal range for my height (I was terribly underweight as well). You don' t have to be much underweight to start messing up your cycles; my periods were normal at 110 lb. but at below 105 lb., I stopped having them entirely.