I've been friends with Sarah since we were in the 7th grade, not BEST friends but always tight. Like 3rd tier BFF, if you know what I'm saying. Anyway - there's a guy in our friend group, Dylan, who I used to be in a FWB relationship with when we all lived in the same town. No one really knew about it, but I'm sure it got around a little. Idc. I moved colleges though, and nearly 3 years passed since I spoke more than a 'Happy birthday' or 'merry christmas' to either Dylan or Sarah. 

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When I moved back home, one of the first people I reconnected with was Sarah. We went to dinner and she and I ended up having a long, 2hr+ conversation in her car about how she and Dylan had been hanging out a lot together a year before, and she'd fallen in love - only to be rejected by him later. I felt bad for her because I thought it was such a long time to still be so hurt over a boy. I was going through a break-up at that time too so we bonded that night, but got caught up in life and didn't see each other again for like 5 months.

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5 months later I'm at a local party and Dylan is there. We're really flirty and touchy and end up making out near the end of the night, and I know some of my mutual friends told Sarah about it. I had totally forgotten she liked him. In my head i rationalized it by thinking it'd been nearly a year and a half since Sarah and Dylan, so hopefully she was over him and didn't feel betrayed by me. I told myself I would have to be super discrete if I rekindled the FWB w Dylan, but we kind of haven't been. Since then to now there's been 2 parties were Sarah will be in the vicinity and he and I are being flirtatious and end up leaving together. 

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In my head I feel like its logically okay and not my responsibility. But another part of me feels guilty and like I should have been more sensitive to Sarah on the basis of some kind of girl-code. AITA?

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TLDR; my close friend told me she had a short-lived thing with a mutual guy-friend who I used to f\*ck during freshman yr of college. He curved her 1 year ago. 6 mo later I f\*cked him again casually, and I'm pretty sure she knows. I don't know what to think or how to feel about my actions. 