What could be wrong that my period wont go away?

Hi guys, I have always had a very regular 28 day cycle no problems with my periods ever! Me and my partner are trying for a baby and I've been off the BC pill from September . They normally last for 5days between start and finish. It came as scheduled on the 11th March and started to go away again 4 days later then 2 days later came back heavier .Since then they have remained here sometimes heavy sometimes light also having light brown blood and little clots. I worried I was miscarrying so went to docs they performed a pregnancy test and it came back negative. She couldn't give me an answer to why my periods wont go away , they have been here from 11th March . I just wondered if anyone else had any ideas and have they went through this too. I'd appreciate honest answers and no time wasters please . thank you