My girlfriend (22F) isn’t comfortable with my daughter (3F) calling her a nickname. How can I tell my daughter my girlfriend isn’t comfortable with it ?

I've been dating my girlfriend for over 8 months. It's been a really good and smooth time.

We see each other 4 or 5 days a week. I also have split custody of my daughter. I get her for a whole week then my ex wife gets her for another week.

My daughter adores my girlfriend and my girlfriend adores my daughter. Buying her gifts and making her sweets. My daughter is always happy when she sees my girlfriend.

A few days ago, my daughter starting calling my girlfriend “mammina Gigi”.

I didn't think much of it but later in the day when my daughter was taking a nap. She pulled me to the side and mentioned that she wasn't really comfortable being called that.

That she's not my daughter's mother or step mom yet. I expressed that she means in a lovey way. My girlfriend told me she just doesn't like it and asked me to tell my daughter to call her by her name.

I agreed but when I went to tell my daughter not to call my girlfriend that. She looked sad and was sad for a while. I think it hurt her feelings.

I don't know if I could explain it better to her.

Any advice ?