Here's one explaination that I found, but there doesn't seem to be much written about it, and it is definantly a factor in joint pain


Ultraviolet A and Ultraviolet B can actually damage DNA near the surface of the skin, altering the DNA and causing the formation of antibodies, thus, creating a lupus flare. Some individuals with lupus report rash, fatigue and aching upon exposure to the sun.

http://www.lupiebin.com/faqs.htm#sun