Could be many things including items in your form, the tune of your bow or the bow itself.
First thing I'd do would be to stop shooting for a while a get it checked out. If you continue with the same duration, form and bow, it could get worse.
At one time I got a case of tennis elbow from a certain bow. No matter how I tried to tame the bow through tuning, grip position, form changes, etc. it still hurt.
What I did was go through some tennis elbow excercises (easily found on the web) and switched bows (sold the old one), and nock on wood, it did not come back. I also had a similar problem with the bow hand wrist with a particular bow, got rid of that one also.