I (20m) have been dating my girlfriend (19f) for about 10 months straight, currently a LDR. 
Recently a very good female friend (20f, goes to my college unlike gf) has been making comments about how much my current girlfriend sucks, how she is too young for me, isn't very pretty, and just negative comments in general.

Now a good portion of this I had written off as joking as that is the way she delivered it but lately the comments have become just plain mean.  It has begun to really anger me because I can't stand to have someone bad talk someone I love so much, even when it is one of my best friends.

Last weekend it came to a head when she drunkenly told me this for a second time after I had just been mad at her about a similar comment from the week prior.  I told her this was not ok and that she couldn't say stuff like that about my gf if she wanted to remain friends.  Our other friends couldn't believe it and commented on how mean she was being.  She then said, "Its not mean if its true!"  At this point I left the party and went back to my house.

Now reddit I need you to help me understand why she would do this over and over especially when we're such good friends.  I know the obvious option is that she has a thing for me as this is what all my friends believe but I'm holding out for something that is easier to hear haha.  

Extra info: Before I was dating my gf I was friends with this girl but basically never went beyond that.  I chose my gf over her but I never tried to make a move on this girl and she never made a move either, so I never really confused her I don't think. This is why its so hard for me to understand that she might like me as more than a friend.
user
Best friend that is a girl constantly makes negative comments about my gf. Why and what
assistant
I (20m) have been dating my girlfriend (19f) for about 10 months straight, currently a LDR. 
Recently a very good female friend (20f, goes to my college unlike gf) has been making comments about how much my current girlfriend sucks, how she is too young for me, isn't very pretty, and just negative comments in general.

Now a good portion of this I had written off as joking as that is the way she delivered it but lately the comments have become just plain mean.  It has begun to really anger me because I can't stand to have someone bad talk someone I love so much, even when it is one of my best friends.

Last weekend it came to a head when she drunkenly told me this for a second time after I had just been mad at her about a similar comment from the week prior.  I told her this was not ok and that she couldn't say stuff like that about my gf if she wanted to remain friends.  Our other friends couldn't believe it and commented on how mean she was being.  She then said, "Its not mean if its true!"  At this point I left the party and went back to my house.

Now reddit I need you to help me understand why she would do this over and over especially when we're such good friends.  I know the obvious option is that she has a thing for me as this is what all my friends believe but I'm holding out for something that is easier to hear haha.  

Extra info: Before I was dating my gf I was friends with this girl but basically never went beyond that.  I chose my gf over her but I never tried to make a move on this girl and she never made a move either, so I never really confused her I don't think. This is why its so hard for me to understand that she might like me as more than a friend.
user
My friend makes negative comments about me and my girlfriend to my LDR gf.
assistant