Hey gay gents and ladies...i need ur help.?

there's this girl at my school who i used to have the biggest crush on but since i've found my girlfriend i haven't thought about her since.my girlfriend and i are very open about our relationship and we're always seen holding hands and that sort of thing.well this girl i used to like is in one of my cles and so today we were talking and she asked me if i thought she was attractive.i told her i used to but i wasn't flirting.she looked disgusted for a minute but didn't say anything else.i left in the middle of cl to use the restroom and when i went to wash my hands she was there.i smiled but didn't say anything and as i was walking out the door she grabbed me and kissed me!i dont know what was wrong with me but i started kissing her back.i didn't want to but i did and now i feel strange because i don't feel any regrets and i should.at lunch we made eye contact from time to time but didn't talk to each other.then today after school my girlfriend and i saw her at starbucks and i tried to pretend like i didn't see her but my girlfriend said hi and gave her a hug then said as she walked away.."she's so pretty and nice.i used to think she was a because she used to always stare at you but...i guess she's just friendly." i feel horrible.what do i do?it's clear we're both attracted to each other but i have a girlfriend now...should i tell her this is where we stop?or keep going?