So there is this girl who I am friends with and she knows I had/have feelings for her, and we hang out a lot just being friends but all of a sudden she gets really jealous when we bring up this other girl Katie. She stopped an in depth conversation just to listen to me and a mate talk about this other girl. What does that mean? She always says she doesn't like me but then she gets jealous!

It's possible that she likes you...but it's also possible that she is just jealous of this particular girl for some reason.

Me? I would ask her why she is bothered by the other girl.

How is it you are sure she is jealous but you don't know if she likes you?
It could be that she really likes the friendship and that is why she seems jealous. Someone else being friends with you. Why don't you ask her?