Ow 
Was it cold when you slept and you didn’t try much to warm yourself? Sometimes aching happens because of the cold . . .
That’s the sign of a sudden cardiac arrest and/or brain explosion
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You should refrain from intense exercise for today in case you hurt yourself.
bad sleep position
forgotten/unnoticed muscle strain
its normal, usually
when your right shoulder starts aching its a sign 3 men in black suits and sunglasses will arrive at your driveway at 9 pm tonight. it happened to me. or that was unrelated
If you haven’t done anything to get hurt or extensive weight lifting with bad technique/posture recently, its probably just a muscle pull