Know you say you can't got to a Doctor, but that would be my first advice to you. The sooner you get on symptoms and early intervention with correct PT, etc. the better chances of a recovery. The longer an injury remains it becomes more difficult to treat and doesn't seem to isolate itself to the site of injury, but spreads.

Best free guesses would be some type of Long Thoracic Nerve injury and/or dysfunction of your shoulder girdle. Shoulders and pelvises are very much related and naturally move together through walking and other activities. You don't mention any neurological symptoms, but things like weight lifting, therabands and other forms of strengthening usually aggravate neuro problems.

You're 19 and have your entire life ahead of you to work and be healthy. Research, research, research the best Doctor you can find and make an appt. A GP, Family Doc or Doc in the Box will not be the place to go. My first guess would be a Neurologist, but the LTN is not an easy one to test accurately. Good luck.