Napping and muscle soreness

Kind of odd thing, but I always wondered, why after a nap my muscle feel sore. For instance, I did back yesterday, sleep 8 hours. Next day wake up, but midway thru day I take a 20-minute nap and back muscles feel tight. This has always happened just want and explanation to what is happening, I hope.
 
And they didn't feel sore when you woke up in the morning?
 
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