Liz,
You might have something called frozen shoulder. This is a condition, not an injury or a disease, that tends to settle on middle-aged women. There is no known cause, doctors appear to be baffled and of varied opinion on treatment. The good news is that it eventually heals itself. I suffered from frozen shoulder a few years ago. In November of 2003, I had what you are describing. Pain in my right arm about 3 inches below the shoulder. Crossing my arm across my chest caused discomfort, removing a jog bra overhead was worse. This state continued for a few months and I just thought I'd pulled a muscle or something. In Jan of 2004 I was still able to mop the kitchen floor (hmmm, don't remember doing that since!) and in mid Feb. I stumbled and threw my arms out to catch myself and burst into tears shrieking with pain. Finally went to a doc who was useless. Tried accupuncture and the guy kept telling me to just relax. I'm not dissing accupuncture, but my "condition" wasn't going to get resolved with it. Finally went to another doc who confirmed frozen shoulder. The soft tissue within the joint develops scar tissue or adhesions. A physical therapist gave me the analogy of slick magazine papers that get wet and then can't slide any more. The scar tissue is too strong to try to break or "work through." Eventually, the adhesions are re-absorbed and the condition fades. I know this sounds weird. It happens in 3 stages. The 1st is the freezing up, and this is very painful. You could be in the early stages of this. Amazingly, the "frozen" stage isn't painful, you just can't use your shoulder. The recovery stage also isn't painful. When I was at my worst, I could raise my arm only about chest high and that hurt. I gave up wearing anything but the loosest tops that could be pulled overhead easily by my left arm. No side zip pants because I couldn't reach over. Couldn't drive my manual shift car, etc, etc. Sleeping was very difficult and the pain was constant. At my wits end, I found a Bowan technique woman and also went to a neuro-muscular massuese. Shortly after trying both of these, I traveled to France (from Houston) and while there, started entering the frozen stage so the pain was diminishing. I'm not sure if any or all of that helped. That was in May. By Sept of 2004, I was into the recovery stage. My doc did not recommend physical therapy but gave me 3 simple exercises I could do on my own. They all hurt and never seemed to help. A phys. therapist told me that he's known people who've recovered faster NOT doing therapy than those that did. It seems highly individual with no correlation to current physical condition. I've always been extremely flexible in my shoulders and had not injured myself. Now, I still have a small bit of restriction that I really don't even notice.

The Mayo Clinic website has a pretty good description of frozen shoulder. I had many people say, "Oh yeah, I had that and just worked through it." Mostly men.   Sorry this is so long. Hope this isn't what you have although, it is probably better than rotator cuff injury and I didn't need surgery. I really had about 4 miserable months and fortunately, I'm a singer not a fiddler, etc.
Best of luck.
Emily