2 periods in one month now spotting

I don't know what's going on with my body. My doctors aren't helping either. May 1st I started a pretty normal period after my period I noticed I was having pain on my lower left side and my boobs hurt really bad usually they stop after my period. So I went to the doctor they didn't know what was wrong. Then on  May 18th I started bleeding again I thought maybe I was just spotting but it was a full heavy period with bad cramps. I went back to the doctor thinking I miscarried because I was in a lot of pain but nothing. Now today I started spotting again and that never happens I'm really confused why I'm bleeding so much! Please help the doctor has checked for stds all negative checked my blood count everything was normal! I need answers!