Another reason may be that she simply didn’t notice you before or thought you wasn’t interested in dating, or a friend may had said that she wasn’t your type because she thought you would shoot her down or wanted you for herself. Or maybe there were things she didn’t know about you till your girlfriend started bragging about you.
Sometimes people will assume that someone isn’t available because they didn’t make their interest in them clear. So they assume that you may be dating someone else. When I was young, I didn’t know that a boy liked me until I started dating a friend of ours. Since we were all friends, he thought I would simply not date guy friends. I didn’t find out till he started dating someone else. I actually liked him the whole time but thought he saw me as a sister, so I never said anything because I didn’t want things to become awkward if he didn’t feel the same. We were friends for years.
I once had a male friend (ex friend) of my husband tell me that he would treat me like gold if I was his woman. I told him that he doesn’t know me, so why would he say that. He said that for 6 months on deployment, my husband talked about how wonderful of a wife and mother I was. He said he had never been so jealous of another person. He said he fell in love with me.
I told my husband to never brag so much again. It will just bring trouble my way.
Luckily, he took my rejection well and said he would not bother me anymore and wished me well.
So maybe your current girlfriend is bragging a bit too much. (You never know how much you want a cold ice cream on a hot day, till you see someone really enjoying one.)