I've recently started experiencing a weird issue... I have a habit of turning onto my back when sleeping, Which is strange in itself, as up until a couple of months ago I have always slept on my side or stomach.
So I may fall asleep on my side, but I'll wake up on my back. In pain! My abs and general core muscles will be sore, and a single intake of breath results in a really sharp pain through my diaphragm (I think) and abdominal muscles. As soon as I sit up or turn back to my side, all is fine, no sign of pain. Also no such pain happens during the day.
But then I'm waking up a couple of hours later on my back, AGAIN.
I have noticed this is worse during DOMS. Although I can't be sure since I've been working out a fair amount lately.