Irregular period.

Last month I had 2 periods. This month I haven't started.... My periods are always irregular. The only time I have missed a period is when I was pregnant with my son. I have taken home pregnancy tests witch all come up negative.. I do however get weak in the stomach at certain things that never used to bother me.. I have been gaining weight, witch could also be a side effect from my medicine. I have also been eating more.. Witch could also be a side effect of my medicine.. I need some help.... Some insight on what might be going on? Any advice would be appreciated.