Implantation bleeding??

So 3 weeks ago I had my period... I have very painful and heavy periods. I always know before I get my period because of cranmps. Yesterday I wiped and there was just a little bit of bright red blood. I put a tampon in because I just assumed that it was my period. When I took it out later that day and it was brownish blood but not much at all so I just put a pad on and there isn't even enough blood to be on the pad. Only when I wipe and now it's light pink and I'm having a little bit of cramping.... pregnancy test come up negative...could I be pregnant and it's just to early to test or is it just a weird prrion or do I need to go to the doctor and get tested for something else? Help I just don't know what to do from this point. Thanks