In reply to EddieBauer:
Lots of causes. Google thoracic outlet syndrome, rucksack syndrome, brachial neuropathy, parsonage turner syndrome, neuralgyc amytrophy.
But basically yes nerve to serratus anterior knackered for one reason or another. I was diagnosed with parsonage turner. But later as nerve stretch injury, level 2 of 3.
It took 4 years for me, and i climbed harder after than before, but it could feel weird on certain moves for years after. Nothing to do but be easy on yourself, nerves don't like stress. Imagine it working (i kid not). Improvement comes in step changes not gradually. And even when it's back working it can go back to winging after a couple of days on. Now, 11 years later, i don't think about it.
A good neuro physio can help. Other muscles trying to take over can get very tight e.g.Levatir scapulae up into your neck.
You must get it checked out properly because it can be a symptom of really nasty stuff. To be explicit, nerve can be damaged by tumour so mri.
Good luck. PM me if you need to but memory fading. Get to the doc.
On the positive side i have met people who have got over this in months. It depends on the level of damage to the nerve. Level 1 is soon recovered from. Level 3 the nerve has been severed.
Post edited at 21:06