What Causes Anal Itching?

I am 5'5, about 240 and have started having anal itching. I noticed it a few days after having had anal sex with my husband. I thought maybe something was just irritated or scratched. It has been about 3-4 weeks since then and it still itches. It comes and goes with no particular pattern that I have noticed.... Can you please help? Any ideas suggestions of what it could be or what I should do?