My friend is being a b*tch but won't admit it

Can you honestly not remember what the misunderstanding was? Something must have happened to make your friend behave the way that she did. Did she hear gossip about you? Can you ask her what happened, calmly face to face. Try to get to the bottom of things.

Are you sure you're in year 13
This seems too childish

How are things now? Did you ever find out how C found out that you "didn't want" her to go (Was it from S?). At the time, did you tell her you never said such a thing?

One of C's friends told her that I told S that C didn't want S to come to the trip even though I didn't even mention it to S.
C is still pretending like nothing happened and we still talk but it's not like how it used to be and C brought one of her friends (M) into our group and me and the rest of our group don't even talk to or know M that well

Basically my friend was meant to take me home before lunch time (we're in year 13 and had no lessons after) one day and I text her the day before if she still could and she said yes. We had a small misunderstanding (which I don't even remember what it was about) in the morning and then 30 minutes before we were meant to leave she text me saying she's going to her bf's house so can't take me home!
For the whole weekend she kept ignoring me and when she did talk to me it was to tell me that I've apparently changed and talk crap about my bf and when she came in on Monday she acted as if nothing had even happened and suddenly brought one of her friends into our friendship group just so they can talk about me while I'm there!