I don't know if this helps but I have run into your problem before were I have strains in my arm. This happens to me when I am playing fast intervals such as octaves were my hand is stretched. I sometimes lock my hand in the position of a octave and forget to relax. This leads to my arm getting tired and me having an arm strain. I fixed my problem by reminding my self to relax when my arm starts feeling tired, this helped my problem a lot.
I didn't really mean that my hand stays locked up throughout the piece, but throughout the octave section I would keep it in that stretched position. I forgot to mention this before but for me, it helps to not only let your hand relax, but also use your whole forearm to move throughout the octaves. (this is especially the case if your playing broken octaves) I am very anxious too and sometimes I have my shoulders up, thats when I forget to play with my whole forearm and relax.
That's ok, I didn't really mean that my wrist truely locks either ! Lot's of years ago I left off playing piano attempting to master more relaxed octaves in the melody and find myself right back there is all. I know my wrist is stiffer than it should be at least in the right hand.On that note, what fingers are you using for your octaves ?