How Can Irregular Periods Be Regularized?

ok ever since i started getting my period, i wouldn't get it normally. i would get it 1 month then get it like 3 to 5 months later. When i started to exercise i was getting it every month. i am today still doing exercise but i stopped getting my period for quite some time, since February. i also took a pregnancy test in may& it came out negative. what should i do?? please help