You could have an infection anally, like an STD, though it wouldn't likely be from this particular encounter as symptoms wouldn't show up that fast. You don't mention if you are having protected or unprotected anal sex, how often, etc., so you'd know better than we would what your risk for that is.
More likely could be hemorrhoids, proctitis, an anal fissure, rectal prolapse - among other things.
In any case, you should get it checked, especially if it hasn't stopped by now.
I hope you get some good answers, and let us know what happens.