My GUESS is that it is likely related to the radial nerve. It winds round the back of the humerus, and goes through one of the triceps on its way down the arm. Any strain to the triceps could easily have a knock on effect on the radial nerve.
The other aspect is the phenomenom of the 'double crush' injury - this is where a nerve becomes more susceptible to reacting to compression/stretch if it is compromised somewhere else along it's length. The radial nerve is particularly vulnerable in climbers, mainly due to our neck postures (poking chin/tight curve in the mid-cervical region), and in the forearm - the radial nerve passes through some very tight spaces as it passes into the forearm. Again, any slight inflammation there will impact on the radial nerve.
you probably need to give it a few days to settle, and perhaps avoid anything too strenuous for a week or so.
Incidently, do you still do the shoulder stability exercises I showed you years back? Triceps will work hard to stabilise the shoulder if the other stabilisers aren't working too well, particularly if the arm is above the head.
Anyway, this is all just supposition...
