Why are my periods so short im 17 please read below?

im 17 have had periods since 12 and im overweight and have spina bifida and my periods are only 3-4 days long and some months they dont come at all im not sexually active at all dont even know any boys besides family and that would be eww! anyways what could be wrong with me...oh and around 1 week before periods that do come i have shooting pain type cramping in left ovary area and that has happen about 2 months now and used not to please help! thanks in advance:)