I need advice. Big time!

So here's what's happened. Last Thursday I had some light bleeding, early for my period, but it happens. i put in a tampon. there was nothing on it except a wee bit of light brown. Then nothing. Next day, I used the bathroom, there was a little red on the toilet paper, I put in a tampon. Nothing on it later. Saturday nothing, so that night i stayed with a friend, we messed around Saturday night, nothing. Sunday morning we started to and I'm bleeding again lightly, but looked like a lot because we were having sex. Since Sunday morning, nothing except pressure and tightness down there. light cramping off and on. I took a test yesterday afternoon and it was negative. There's a high chance of pregnancy, but I don't think I can have kids because my ex husband and I tried for 5 years and nothing. Is there a chance this could be an STD? If so, what? my period is always heavy. has been since i first started at 11, so I'm very sure it wasn't just a light one. this is incredibly abnormal. even the bleeding was kinda watered down looking, but still red. i just need all kinds of input. also I'm 3days late for my period.