Why does my friend seem jealous when I talk to girls?

Hi, I’m a bisexual woman in my twenties. I’ve had relationships with women and men, though I’m currently single. My most recent relationship was with a woman and it lasted for nearly two years. Anyway, this friend - we’ve been friends for 10 years now and sometimes we go out to bars together with other people. I do consider her a really close friend. This friend is 25 and has actually NEVER had any relationship before of her own, and has never been on any date. She told me she’s straight, but we rarely talk about dating because she gets very awkward about it, so I don’t raise the topic anymore. This friend once told me she has never felt attracted to anyone, and sometimes I wonder if she’s asexual.

When we go to bars, this friend is very CLINGY towards me and will try to engage me in conversation for the WHOLE night in an attempt to prevent me talking to other new people (at least that’s how I feel). For example, last weekend we were out with mates, and I saw a beautiful girl nearby who I wanted to approach. I began talking with this new girl a little, and my friend looked quite pissed off - she later wanted to know what I was saying to her etc. I just felt uncomfortable trying to approach this cute girl, because my friend kept looking with this blank expression and it made me feel weird. This happens almost every time we all go out to bars. Then, for a long time after we’ve been out, my friend won’t message me or will be really short with her replies. I don’t know what she thinks and I don’t know how to approach the topic. Thoughts?