Hi B.,
I used to be a massage therapist and it does sound like you've got something out of place. You may be able to adjust it yourself with a couple of simple stretches. If you can't feel near-immediate relief, then you need to go see a doctor and chiropractor.
First rule of thumb is that you'll often feel the pain on the opposite side of where the tightness is, so you have to stretch both sides. If you have a muscle that is extraordinarily tight on the right side, it pulls and can cause the pain on the exact opposite side (like a mirror reaction).
Based on your description, I have two suggested stretches:
1. Side stretch. Do both standing (feet shoulder-width apart) and on your knees. Keep your back straight, lift your arm straight up and bend sideways at the waist, reaching through the palm of your hand as far as you can. This will help relax the intercostal muscle between your ribs. Hold for at least 30 seconds and repeat several times a day.
(This one sounds complicated, but it is the best stretch for the muscle running under the shoulder blade and offers the greatest relief)
2. Stand sideways to a kitchen sink, feet slightly wider than shoulder width apart (do left side first). You'll be about 1-2 feet away from the sink depending on how tall you are. Rotate your upper body left towards the sink and grab the edge with both hands, bending left knee slightly. You're going to mimic a push/pull action now. Bending knees deeper, push on the edge of the sink with your left hand and pull with your right. If done correctly, you should feel a *deep* stretch underneath your shoulder blade running from your shoulder and possibly down to your lower back, depending on how tight the muscle is.
If you don't feel an almost immediate relief from the pain you're describing, then you should get to a doctor to rule out any medical issues and a chiropractor to rule out any structural issues. If you do feel some relief, continue with the stretches as often as needed until you don't have the problem anymore. Stretching can be an effective way to help but can take some time to last if the muscle is being stubborn.
Good luck and let us know how it all works out!