Confused & Concerned

I haven't got my period since October 2016. In November 2016 I took 2 pregnancy test and they were both negative. Either in January or February I went to the doctor and took a test there and it was also negative. Doctor didn't know what it was. Recently (March) I took another test and it was also negative. On April 13 or 14 my left nipple started to hurt. I was thinking maybe because just a few days ago the left one itched and it might of got a cut or maybe I was beginning my period again. But now on both nipples hurt when I touch them and no period. I have no clue. I've read that it could be Mastitis, Breast cancer or anything possible. HELP