What causes irregular periods?

Bleed three months straight; not sure what happened, nor my doctor, but I looked into it everything that could be wrong, and everything came back okay, even a blood work pregnancy test (negative). So in the first week of April, the bleeding finally stops, and I bought test after test with saying I am not pregnant. Seen a clinic two weeks ago and said I’m pregnant, but blood work results came back negative, and I have to do another test in a week, honestly not sure if these kicks I feel are real or not. What I’m really asking if other women had something similar happen to them….