What Causes Bleeding From The Anus?

This is embarrassing but, I had sex at the beginning of last month, and like an idiot, I didn't use contraceptive. I did ask him if he had been tested, he said he was four months ago and was clean. But he could've caught something in that time, right? I'm freaking out and think I might have contracted an std. Either that or I ripped by butt whole. I do remember being constipated about a week ago and was on the toilet for awhile. Now when I wipe by butt after using the toilet there's a little bit of blood. And my butt hurts a little. And it hurts less when I'm not sitting. Am I freaking out for no reason? Either way I will be getting tested.