No Period In 3 Months and I'm Not Pregnant .?

Hi, i'm 18 . My last period was on 1/15 . soon after i found out i was pregnant & got an abortion on 4/5 . the doctors told me my period should be regular in about 3-8 weeks . but nothing never came . i took about 9 pregnancy tests & all came back negative . my GYN doctors says everything looks normal . i've been having abdominal pains & headaches here & there . what could be wrong with me .