Lover's Cramp

The uncomfortable, and sometimes painful, seizing up of the muscles in the hand and forearm caused by excessive stimulation of another's or one's own genitals, normally from rubbing, stroking, or fingering.

"Man, I spent so long fingering my girl last night that I've developed Lover's Cramp"

"Sorry, I can't play the guitar today, I've got Lover's Cramp"