Possible Exposure to HIV - what are the odds?

I was with someone about five months ago now. I found out later that he slept around a lot. I went through the full series of STI tests, including the HIV test, about a month after the incident. I found out a couple days ago that the girl he was with before me, who I'd been friends with and said she hadn't slept with anyone else, has now tested HIV positive. I have my next test in about a month. How worried should I be, and what are the odds that I actually have HIV?