Throwaway for privacy

I (16m) have a friend (14f) who got her braces tightened on Thursday. On Friday during lunch she asked if someone if someone could bite her apple because "the first bite is always the hardest". We've been friends for a bit over a year now and I didn't think it would be a big deal if I bit it for her so I took the apple and bit it.

My girlfriend didn't say anything in the moment but during her study hall she cussed me out over text about me being too "friendly" with other girls and how I should've let someone else at the table do it. I apologized about and explained how I didn't think it'd be that big of an deal because as a group we share food and drinks constantly (like to the point where if one of us is sick then it's almost guaranteed the rest of will get sick). She explained how it wasn't the fact that we were sharing food it was the gesture but I still don't get it. I don't get how there's a difference because it's legit the same thing to me. She's still mad at me and refuses to explain how it's different.

AITA?