Hi... Well I Started Bleeding Last Month For 3weeks.. Constantly.

Hi... Well I started bleeding last month for 3weeks.. Constantly. I'm only 22yrs old ... I've been trying to get pregnant somewhat. But. This happens ... At least 5times out of a year... My period be really dark almost black with blood clogs. I go to the doctor about it.. But they just tell me it's irregular bleeding... I have been stressing emotional. It causes back pains and stomach aches. I'm just wondering.. How can I prevent this from happening ??? Or what would be the cause..?? I'm worried and my family.. I do it so much to where I even get use to it, well, I started 3 weeks ago and started right back