What else could make you miss your period other than being pregnant?

I joined the gym about 4 months ago to lose 20 lbs and stay fit. About 2 months ago, I started taking pilates and kickboxing at the gym also. Now my period for this month never came. I took 2 pregnancy tests just to make sure that it said I wasn't pregnant. So my question is...what else causes you to miss your period? could it be the gym?.