I'll play the nerve compression card just to be different and say that as you did your pull up, you tilted your head back ever so slightly at the top of the exercise; the neck extension plus the stabilizing contraction of the levator scap muscle pinched a couple nerve roots on the affected side sending radiating sxs into your arm/hand and creating spasms in the suboccipital muscles (or maybe cervical facet restriction) which will feel tight when the neck is bent forward; treatment options i'd look into would be soft tissue work to loosen the neck and upper traps, cervical distraction to stretch cervical muscles and possibly decompress nerve roots, rest, ice and ibuprophen to calm nerve roots; upon return to gym be sure to keep neck in a neutral position during all exercises. Good luck.