Why Am I Having Irregular Menses?

Hi i am 21 year old and from past few my facing the problem of irregular periods. as i was having some medic ens for my hear pain i neglected it. But i have skipped my period in july and now its almost august still i have not got. i have consult one doc he recommended me medicine but still no use. And even i have redued my weight from 55kg to direct 50kg from Jan till now. Plz help wat should i do.