I haven't gotten my period in 78 days and i ve taken 6 preg test and all r neg wht can it b?

my last period started march 22 and ended march 28 i have taken 6 pregnancy tests with 4 different brands i used clear-blue, first response, answer, and equate and every one of them said negative. im only 22 so i no its not like menopause or anything but i dont no what else it could be ive been trying to have a baby for years now and still haven't. and this isn't the first time this has happened to me last year around this same time i didn't have my period for 7 weeks and the same thing happened then all the test i took were negative. please if anyone has any clue as to what i can be please let me no thank you so much.