is it a sharp pain going from edge of one shoulder blade to the sternum, like someone
is pushing a very hot, very dull, butter knife through your body? hurts when you take
a deep breath, also can hurt to lie down?
you should check it out, but i suspect it's a pulled muscle or muscle fatigue or whatever
you want to call it. you say you did nothing strenuous, which is not the same as
doing nothing at all. what did you do the day before? housework, cooking, writing,
moving a couple small boxes?
since you're not healed yet, especially with an unfused sternum, you're holding your
body in an awkward manner. your posture is going to be abnormal, putting more
stress on the muscles holding you off-center.
what helps for me when this happens: pop a pain pill (only if severe pain), take a
really hot shower, massage if you have an assistant, do some stretching exercises.
slowly stretch the hurting muscles where it hurts. this part sounds masochistic, but
a couple stretches usually clears up the tightness for me.