I get that all the time, like you said its mainly due to poor posture. try to straighten up as much as you can, rolling your shoulders back & down.
You could get a heat pack or hot water bottle on the area to loosen the muscle, and then if you can manage it, get a tennis ball or something similar, lye on your back and roll the ball under your shoulder. Might not be very comfortable but should really loosen everything up. if you cant do that get someone to give you a massage.
+ obv work within what your allowed to do with the new additions 🙂
hope that helps. im a qualified sports massage therapist so see this all the time 🙂