I get this sometimes...I attribute it not only to my activities today...but the day before and the day before that and the day before that, etc. Every day just getting out of bed and facing the day takes so much effort and sometimes I just hit that limit where my body says, "NO"...and no stubbornness on my part can make it move. So then I rest and relax...take it easy...and tomorrow continue on as usual. If you are concerned about it...check with your doctor as it could be something else...but this happens to me a few times a month depending on what exactly I have been doing...happens more often when I'm not sleeping well.