I've been playing guitar for about a year and a half now and I usually play sitting down but when I play standing up my wrist (on the hand I use to fret) starts to ache after about 10 mins.

I always do warm ups before I play as well.. is there anything I can do to stop this?
Same thing happened to me when I first starting playing bar chords you just gotta work up the strength in your wrist I guess. It doesnt really happen to me anymore unless I play for hours.
make sure yuor guitar isnt too low, theres lots of finger strengthening exercises yuo can do to make your hands stronger. Also, dont push yourself too much if you feel pain then put the guitar down and get some rest.
Yeah, my guess is your guitar is to low, I had the same problem, but when I raised it, it all got better.