I sit next to this person in one of my lectures and for a few weeks now she's been getting progressively more insane.

Today for example, she talked about sometimes wanting to kill people like virginia tech and then started to mimick me, repeating what I'd said in various tones. At the time we (me and and a fellow student) kind of laughed it off and said "get a gamecube" or something, but now thinking about it in the safety of my bedroom, it was all rather bizzare. I've known this person for about a month, and see them twice a week in the lectures.

What do I do?