Did I fumble *too* hard?

I self diagnosed myself with mild social anxiety (could be wrong but I have some symptoms) anyway, in school break there is literally only 1 bench to fit 3 people and since no one was sitting I came and sat on the far right, after some time a friend group of 3 girls came to sit but since I was sitting there only 1 of them sat down on the far left, after a minute one of them sat right next to me which surprised me because in our culture its not usual for guys to sit next to girls that close , anyway my immediate thought was to get up but I felt like I would be rude and seem like I don't want to sit next to them and as usual I over thought it and stayed seated, we were all watching the others play football infront of us and after like 10 minutes she opened a pack of biscuits and she offered me one...and as the stupid person that I am I wave my hand and head as a sign of "No" out of instinct, and I didn't even thank her for offering..and the thing is she BARELY knows me the only interaction we had i didn’t event talk to her she just borrowed a book for a minute to see an answer and that's it, at the time I just thought she was being generous and tbh I still kinda think so.

What do you guys think? Was this a universal fumble? Should I have took the dam biscuit so I don't embarrass her?