Happens to me All the time. If you have strained/bruised a muscle, laying down does Not ensure it will relax into the correct alignment with the other muscles it interacts with. That means things may pull where they shouldn't, and cause some pain. MY best advice, is take a baby aspirin, and for for a walk. Walking makes major muscle groups pull back and forth, tense and release, and seems to help get things back into their correct situation, Helps me.