Sympathies for having some new problem show up that wasn't bothering you before. Haven't had that particular problem, but have had others. Sometimes they go away by themselves without my being able to figure out why they happened.
These are the most common causes of arm slap:
1. Bow arm forearm rotated sideways. I see this more often with women, but it happens to men occasionally too. Hold out your bow arm like you're ordering someone to "halt!" with your hand 90* up from your arm and your fingers pointing up. Keep the same forearm position when you draw the bow.
2. Bow hand too far into grip. The weight of the bow should be on the part of your hand between your lifeline and thumb. No part of the bow handle should be further into the palm of your hand than the lifeline.
3. Brace height too low. Check your brace height and make sure it is at the same height it was when you weren't slapping your arm.
You also said your accuracy was suffering. Do an overall form check, including your bow arm shoulder. Make sure your bow arm shoulder stays down as you draw the bow.