Sounds as if you need to talk with your ortho doc about the possible causes of the pain, could be lots of things. My ortho doc advised that I never get out of the saddle to pedal because, in his words, that places about a 100 times more load on the knee than sitting while climbing." Now, maybe his numbers are off, but the general message was that standing to climb is likely to wear more quickly the polyethelyne component of the prothesis.

I do get out of the saddle to take a positioning break and I sometimes get out on a hard climb, but I then use the easiest gear I have and limit revolutions and how many times overall I do that during a ride. I would ask your doc about the potential consequences for your prothesis if you stand while climbing, assuming you get the pain issues resolved.