I had something that sounds similar. I ended up going to a physio about it. After a couple of visits, and some manipulation, we put it down to how I slept at night: I tend to sleep on my side, and at the time was only using one pillow. As I am quite broad-shouldered, this put my neck at an acute angle, which was putting pressure on the nerve that down down my back into my arm.
It got to a stage where I could barely get out of bed in the morning
Cure: To sleep with my head on two pillows, instead of one. This pretty much cured it overnight.
Moral of this story? Perhaps it's something to do with the angle you hold your neck at during riding and sleeping at night.