This doesn't sound like an STD. You don't say whether your anus has been sexually exposed in any way, but even if so, it sounds more like an injury or a hemorrhoid, perhaps with a secondary non-STD infection. The main thing is to get professionally evaluated; see your doctor as soon as possible. Not an emergency, but try to see someone in the next day or two.
i posted a few pix on my profile... but thanks for the info... i'm really scared...