Was it the belt buckle
that rubbed on
? Often the metals can cause skin
irritation, Urticaria as Smilerdeb mentioned, Nickel allergy is very common, also. If the belt
was leather, there are tannins in the processing that can cause a welt. If it was a plastic based one, then again, it can cause welts.
The fact your knees are so sore and red, makes me think you need to
get some anti-bodies blood tests done. The way thyroid anti-bodies affect joints is quite well known.