You mentioned that now the opposite side is involved. That's to be expected: above all, the body wants balance. So, hypertension on one side of the body will eventually create hypertension on the opposite side. I take it that you are seeing a massage therapist as well as a physical therapist?

Hard to make a diagnosis from a photo. But to me it appears likely that your subscapularis contains very active trigger points. Not only will they refer pain to seemingly unrelated areas of the back, but the chronic contraction at the lateral border is pulling the scapula forward, creating hypertonicity in the rhomboids and probably in the posterior superior serratus. Regardless of the etiology, it's a virtual certainty that you now have active trigger points all over your back...in supraspinatus, subscapularis, teres minor, posterior serratus superior and inferior, anterior serratus, infraspinatus, the rhomboids, the traps, and in the TOS group (i.e., all three scalenes, pec minor and perhaps even in the coracobrachialis).

Richard D may have surmised the original source of your problem. Exalted by your great performance in the meet, you went to bed still excited, tense, and slept very soundly. Do you perhaps sleep predominantly on the problem side, perhaps with that arm extended forward? If so you could have unintentionally limited blood flow to the area when it needed it most. Then when you awakened, tight and experiencing the burning pain that sometimes results from neuromuscular compression during deep sleep, you logically performed your stretching routine, and inadvertently made it worse.

If you aren't seeing a massage therapist certified in neuromuscular therapy and myofascial release, my advice is to do so ASAP.