Re: Shoulder Pain following sleep
It's possible that it can be multiple things. Tendonitis comes and goes with exercises and rest generally.. so unless you just suddenly stopped working out and it goes away then you started again and it comes back then it's something like that. Generally, you can tell tendonitis apart in that it's not necessarily sore or anything but it's close to the bone and doesn't feel like muscle. e.g. at medial epicondyle is easiest one to check for as you already had it. Since it's not really the same which would've been my guess I don't think it's tendonitis.... although it could be a bit of tendonitis and something else.
To be honest, it would probably be good to have a PT look at it if you have lots of questions.
Sleep is like you mentioned though. I would try to learn how to sleep on your back and stay on your back. I tend to roll and it tends to mess with me a little so I'm trying to fix that.