Do they have testing that proved tendinitis or are they just calling it that?
Do you have any neck pain or tightness in the upper back at all?
If so you might explore our thoracic outlet syndrome forum (TOS) for short.
http://neurotalk.psychcentral.com/forum24.html


With all the activities you have done were there any falls, hits or injuries?
If that is a possibility--
You might consider seeing a few expert & skilled chiropractors??
If nothing else to get opinions?
We can easily get out of alignment and that will strain our muscles & tendons.

The chiro I found used many modalities as well as adjustments - low level laser, IF stim, ultrasound.. wasn't painful or scary at all.

He helped my issues much more than the many PT places I went to.
RSI/TOS with some chronic myofascial issues..

TOS is often not well known by most MDs, so could be cubital tunnel/golfers elbow etc - or might be TOS, or TOS like if the cause/compression is higher up in the neck/collarbone areas. If hyper mobile that can be a factor too.

And if the spine (also c1/c2 ) has been misaligned for quite awhile the body will try to rebalanced itself by tilting hips, shoulders etc and that can cause upper & lower body pains..

www.upcspine.com

But all the imaging & blood work is often needed to rule out those solid testable things first.
But it doesn't hurt to have the mechanical side of things checked into.
Most MDs won't mention that.. unless something is so obvious that it shows visually or on imaging..