I get the same problem occasionally, and by self diagnosis I think it's a degree of pressure between the two heads of FCU, which I assume may be more likely if you climb (they get bulkier). It only bothers me occasionally and I know what the treatment involves, so I put up with it.
I still have a numb area on the left hand after I had superficial radial nerve entrapment. My God, that was pain that I never want again.