Am I wrong for saying no to a favor my coworker ask?

i work at a veterinary clinic and have been there for about two and a half years. i genuinely love the job and for the most part the team is good but there's this one coworker Simone who i've had an off dynamic with pretty much since she started about a year ago. nothing dramatic, just one of those situations where two people don't quite click and you both know it and just kind of work around each other.

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two months ago i found out through someone else on the team that Simone had been telling people i was hard to work with and that i had an attitude problem. not to my face, just quietly to other people in a way that had apparently been going on for a while. i didn't confront her about it because honestly i didn't know how to bring it up without it becoming a whole thing and i just let it sit which i know wasn't the most mature call.

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last week she came up to me and very casually asked if i could swap my saturday shift with her thursday because she had something personal come on and really needed the weekend free. and she asked in this tone like we were totally fine which i guess in her mind we were since she probably didn't know i knew what she'd been saying.

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i said no.

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i told her i had plans on thursday that i'd already made and couldn't move and that i was sorry i couldn't help this time. the thursday plans are real but they're not unmovable if i'm being honest and i think she could tell i wasn't being completely straight with her.

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she went to our supervisor saying i was being uncooperative and our supervisor pulled me aside and asked if everything was okay between us and i said yes because i didn't want to get into the whole backstory at work.

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now two people on the team have mentioned that Simone seems really stressed about the weekend thing and i feel a little guilty seeing her stressed even though i'm still pretty hurt about what she was saying behind my back.

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my friend outside of work says i'm completely justified. my sister thinks i'm letting ego get in the way of just being the bigger person.

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am i wrong for saying no?