The nurse told me heating pad but just keep it away from belly. If you Google pubic symphysis disfunction there is a video on self treatment that helped a lot. I'm only 36 weeks and have been going to the gym until Tuesday... I hope this isn't here to stay!
I still use heatpacks this is my 3rd pregnancy. I limit how hot and don't put it on my belly. I have been known to sit on many times. :-)
I'd sit on an ice pack :) I have pretty bad vagina soreness too and I think cool would feel better than warm. I'm sure warm is ok just as long as you don't sit on it too long.