Nicole:
This is definitely OCD. It's an intrusive thought that seems irrational but just won't go away, and that gets worse when you try to argue with it. I've never had this exact worry, but I've had stuff like it. Remember, just because you have a thought doesn't make it true. I often do sort of "magical thinking." I think that if I can't convince myself something isn't true -- in your case, maybe it's the thought that you've contracted HIV -- then it will happen. But it won't. The thought that you might have HIV will not hurt you. So try not to fight so hard to banish it from your mind. I know that is really difficult. I hope you start feeling better soon, and I'm pulling for you.