You should go get a blood test done to conform your pregnancy. You really shouldnt be using tampons if you may be pregnant. Good Luck!!!
How heavy was the bleeding? Depending on how long since your last period or when you ovulated it could be implantation bleeding. And was the negative after two positives or before? It sounds like it could of been implantation bleeding, when i had mine, it was just a very little bit on the paper after i went to the bathroom (sorry if TMI) i thought i was starting my period, put in a tampon when back an hr later to check and nothing. about 2 weeks later found out i was prego. You could always make an appt with your reg doc and tell them about it and ask for a blood test.
If you suspect you are pregnant and are bleeding do not use tampons. They can cause toxic shock syndrome (TSS). Use sanitary pads only.
If you had positive tests and are bleeding with negative results now it could mean chemical pregnancy (miscarriage).