While I can't say ABSOLUTELY it caused it, if I had money to bet, I'd bet it. Sun (and stress) are the WORST things for lupus, and can indeed cause a flare. This disease makes us very photosensitive...too bad it doesn't make us photoGENIC! (My face is beginning to look like Jay Leno's because of all the bloating, lol)
Always, always use sunscreen, and wear a hat. I avoid the sun entirely.