Hard call. Consider fatigue in muscles not used to that level of usage, answer rest, either no surf session, or severely shortened session, secondary a review of paddle technique, would be helpful. If you are still a novice surfer, then technique would be the most important item to review.
Really bad technique puts stress on the elbows and triceps and up to the origin of the biceps.
Usual cause of problems is not stroking along the board but spreading the arms wide out. This puts more rotational forces into the rotator cuff area, irritation and inflammation occur.
Rest, massage by someone who knows what they are doing, there may be impedance of the biceps tendon ( clear diagnosis and treatment by a chiropractor, myotherapist, physiotherapist any of those but not a backyarder.
There are medical treatments too, be aware though that treatments like cortisone shot will often appear to fix the problem and they help with rest, but symptom relief and then surfing on top could risk more serious structural injury.
Get your problem correctly diagnosed and then choose the treatment accordingly!
Good luck, hopefully just over use
I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷
