Are you a girl? If so, it could be menstrual cramps. Otherwise, it could be something you ate giving your stomach cramps, which will most likely result in diarrhea, but afterwards you should feel better. If it is on the left (I think, but it could be the right) side of your lower stomach, it could be you appendix. Test this by lying flat and then pushing in, if it hurts when you release your fingers, it is your appendix. If you were exercising your stomach in the past few days, it is just sore muscles. If this persists, you should call your doctor.
