Most arm slap is caused by bad form unless you have one of those arms that looks like it should be on the other side of your body
The first place I would look is your grip. You should hold the bow with as little of the heel (pinky side of the lifeline) of the handtouching the bow as possible. Heeling the bow causes a lot of torque and when you release, the bow twists in your hand and the string smacks your forearm. There may be other causes too. Get someone who is a good shot towatch you shoot and check out your form. Remember, the only dumb question is the one you dont ask. There are a lot of guys on here who would be glad to help you
. Ive been shooting for 46 years and I still ask for help.
Good luck

GGBH