Hi. I've Been Having A Really Strange Period. I Spotted

Hi. I've been having a really strange period. I spotted a brown and black discharge about 5 days before I started to flow. Now I'm having a horribly heavy period and woke up last night with severe cramping all over in my lower abdomen, flanks and lower back. When I went to the bathroom I could tell that I had leaked through my tampon, however when I checked it was full of this watery like discharge. This has been so strange and I'm worried it could be an indication that something more serious could be going on.