4 periods in the last month and 3 diffrent birth controls

I have been on the same BC since i was 17. I am 20 now and for the past month and a half have had 4 diffrent periods. I went to the doctors and she told me that my body has got used to the birth control i was on so she switched it. she made me wait to start the new pill until after the bleeding had stopped...once i started the new pill about a week and a half later i got another period i at first ignored it thought it was just my body getting used to the new pill but i started to get dark red blood clots and my cramps got horrible so i went back to the doctors and she switched it agian...its been about 8 days since i started the new pill and i woke up this mornin with horrible cramps and this nasty thick brown blood on my underwear...I am starting to get worried that there is something serious going on...can anyone help me out please??