Hi, I Am A 20 Year Old Female Who Has

Hi, I am a 20 year old female who has been on the pill(28 day/placebo pills) for over 6months before that I was on the injection for 5 years. My last 2 periods have been odd, the 2nd to last one lasted 3days and was only light spotting & came a week earlier then normal. The last period only lasted 2 days then vanished which is odd for me. This morning when I stood up after waking I had this gush of water come out, I was wearing a liner and when I went to the toilet it was completely soaked with pink fluid. Then when I wiped this big blood filled glub came out was about 5-6cms and round. Since then I have only had pink discharge slight nauseous and tired. I have taken a pregnancy test and it has been negative. I have not been pregnant before. Any ideas for what could be going on.