What Is The Treatment For Irregular Periods?

Hi my name is Ashley I am 19 yrs old. For thepast two years I have experienced a lotof missed periods. I was on the pill but that was long ago. I would like to know if there's anything wrong because as of today I am still having missed periods about every other month or so. Also I recently missed my period and now my breasts are feeling pretty heavy and full and they have never felt that way, my nipples are a little sore an painful to touch. I would like to know the causes of all this. Thanks