I think jealously was part of it, but it had more to do with what she perceived as her own short comings rather than Xander being involved with someone else. She was the girl who had been friends with Xander forever, she was the one who once had a crush on him yet he picked Faith; the new girl again. That probably hurt her self esteem some...left her wondering why bug teacher lady, Inca Mummy girl, Cordy and Faith all caught Xander's eye when she didn't. The fact that she thought something like this would never happen when it did also made it clear to her that she didn't know Xander as well as she thought. Add that to the jealousy she had been feeling about Faith and Buffy hanging out and it's understandable why she would react so strongly.