Well, first, I have to admit that I find it unlikely that it was an “accident”. I mean, it is possibly for that to happen accidentally, but very unlikely, especially since it was such a well aimed “slip” that made you bleed… It sounds like a guy trying to pass off what he did as an “accident” when, really, he was going for it.
I don’t know this for certain, I wasn’t there, and I’m not saying an accident is impossible, but the skeptic in me really questions this. You should think a little more about what happened and consider whether or not you need to have a discussion with your boyfriend about experimentation and realize that there could be a MAJOR consent issue here.
Secondly, the rough entry without proper lubrication and foreplay undoubtedly caused some anal fissures (tearing) and that’s why there’s bleeding. It should heal on its own, could take a week for it to heal fully, but if you continue to experience significant bleeding/pain after 24 hours, see a doctor.