The anus can be a pretty sensitive place and prone to bruising and tearing, especially when lube isn't used. Those are things that will usually heal on their own... there is a bit more of a risk with tearing that it could lead to infection (as with cuts elsewhere on the body) so it's worth it that you're paying attention to it... hopefully everything just heals. But if it carries on for a longer time or starts getting worse it might be worth getting checked by a doctor, but it doesn't sound like that's the direction you're going in right now.