String slapping forearm?

[Type1Daddy]

Yep. DL is too long, or your grip is bad.

[huntingpj]

It was fatigue. Shot fine this evening.

[muzzyman]

Try not to rotate your forearm so much inwards. I was having the same problem and it has helped me tremindously

[ZZ Pops]

Just curious...
You experience fatigue at shot #13, was this primarily due to shooting the bow, or were you tired when you started shooting? If it is mostly fatigue from shooting, your draw weight must be too high?

[EarleyBird]

That $#!¥ hurts!