Your probably either playing 2 much........ (which I doubt cuz I play for hours everyday and its happened to me then like twice) or your straining your hands due to stretching or lack of

"form"

Now the thing to be said here is there really isn't any proper "form" in guitar playing. If the way you are playing isn't feeling right then you have to try something new.... Try coming up with it on your own.... or look at videos of the pros playing..... also there are many books.... like guitar for dummies....... that give you some decent advice