Nope my muscles don't tense up at all . I can easily fret a few notes and chords . Granted I sometimes have to position my hand.

As for when doing other activities I've never had this problem. Playing sports
lifting things etc etc . I still think it may be the way my guitar is position or perhaps there could be a problem with my strap.

But yeah its' really frustrating when you wanna play for like even 30 minutes and often have to quit within like 5 :(