Why is my friend so snide and bitter?

I saw an old friend over the weekend and she made a remark about how things haven't changed. She was like,"Oh I see things haven't changed..." It really upset me because it really isn't true- I moved away, am attending graduate school and got a new job. She's still living in the same town we grew up in and working at the same place since college, except she hangs out with new friends. (She left our group and seems to only come back to us just to make snide comments like this one.) I don't get why she's like this or why she's so bitter/resentful. I know it isn't worth it though because I hardly see her, but it still upsets me. Does anyone else have a friend like this?