Something odd can anyone explain?

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Hi there! Today i met with one of my best friends and as usual we made a skandenberg session together. Up until today i used to always beat him with a little effort on my left hand and no effort at all with the right one. Today we started out i beat him once again around 3 times and all of a sudden an annoying pain started in my arm and from then on i felt like i didn't have any strength at all. I couldn't even beat him with my right arm. What happened can anyone please explain me? Oh and im not gealous of him if some would think about that he's one of my best friends i could never be gealous of him i just wanna know what the explanation might be. Thanks a lot! :)