Could anybody help diagnose this skin problem?

I have a large patch of red skin on my lower back with hive like things. Its gets very itchy when the skin gets warm under bedding or clothes. I've also been having a similiar thing happening to my knees, which when they get warm have a feeling of pressure inside them, but they are not itchy. I have to put a cold cloth or an ice pack on them to make the itching go away. Ive been using sorbolene creme on them at present, but it doesnt seem to be helping much.