You probably need a doctor to answer that for you.
Is it a tight squeezing pain; a heavy pain like a heavy weight has been pressing down on it; is it muscular or more internal; does it hurt to expand your lungs when you take a full, deep breath...
There is a myriad of possibilities.
Or, perhaps, in the evening you're roughhousing with kids or grand-kids, and the effects from their elbows and knees dropping on you are just being felt in the AM ;)
Is it a tight squeezing pain; a heavy pain like a heavy weight has been pressing down on it; is it muscular or more internal; does it hurt to expand your lungs when you take a full, deep breath...
There is a myriad of possibilities.
Or, perhaps, in the evening you're roughhousing with kids or grand-kids, and the effects from their elbows and knees dropping on you are just being felt in the AM ;)