Wrist Pains

I'm getting a lot of pain in my wrist lately ive been playing for years but only what you would call "serious" practice for about 6 months and I still get an almost cramp like feeling in my wrist mainly from Power Chord playing but also if im doing legato.

I was wondering if there was something in my thumb position, with power chords I have my thumb against the back of the neck with my first finger playing the root note and muting the bottom unused strings.

Could this be the problem if so how else should I be playing Power Chords.