Does your gun close hard, or do you favor some muscles when you close your gun? It can also come from reloading or sleeping on your arm. When your extend muscles not normally used in a particulair fashion, aches begin.
Does it hurt when you extend your arm in front of you, raise your middle finger and press down on it with your other hand? I was having pain in my elbow, my doctor did this and I came off the exam table. Turns out I had tennis elbow. Give it a shot.
Tennis elbow. Or in this case, trapshooter elbow. Go to the pharmacy and buy one of those braces that are like an elastic strap which fits just below the elbow. What causes the pain is a bit of detachment of the muscles just below the joint. This then gets inflamed. The brace will cure it. Jake