Was the weight gain somewhat unexplained? If so, you may have a thyroid problem. That would definitely cause a change in weight and menstruation.
i agree with sares. probably hormonal. i'd see a dr. to be evaluated.
It sounds like you could have some sort of hormonal imbalance, things such as polycystic ovarian syndrome (which is from hormonal imbalance)cause the exact symptoms you describe. A doctor can do some blood tests to test your hormone levels to give you an exact diagnosis. Polycystic ovarian syndrome causes you to stop having periods or have lighter ones, weight gain and sometimes increased hair growth and acne. Good luck, hope this helps. Sares