One time I went to practice serves and on the first warmup serve (not even half speed) it felt like a large knife went into my shoulder. Excruciating stabbing/burning pain.
The pain went away quickly, but my shoulder was stiff and sore for several weeks. I tried all the usual self remedies: NSAID, gentle stretching & rotator cuff physical therapy, etc. No change at all. The soreness always got worse at night and started going down my arm with numbness. Seemed like impingement in the shoulder.
I finally went to see the top shoulder surgeon for sports injury in my area. After some shoulder/neck x-rays and a 20 minute exam, he said my shoulder was 100% healthy! No bone spurs or abnormalities of any kind, no sign of inflammation, etc.
The problem was a pinched nerve in my neck, between 2 locked vertebrae. Weird, because all the pain & stiffness was in my shoulder/arm and my neck felt completely normal. After some light physical therapy, and a new pillow
, all pain & numbness was gone!
Moral of the story: see a doctor, it could be good news.