a secret affair

hey there, so this is a weird thing i need to get off my chest. its been a few months since it happened but i keep thinking about it and wondering if im the asshole here or not. ive been dating josh for like 5 years now, hes great and all, but lately things just feel kinda stale. then last month at work this new grad student starts coming to my office hours, she was super into our class stuff and really engaged during lectures. anyway, i dont know what got into me but we started chatting more between classes and somehow it turned into us going out for drinks one night after a lecture. next thing you know we end up at her place talking about the coursework and before i knew it, well, lets just say things got heated. we kissed and yeah, it went further than that too. but here comes the weird part - afterwards she tells me shes been seeing someone else this whole time but thought they could be more of a friend with benefits kind of thing. now im left wondering if i was just another fling for her or if she was really into me too? and what about josh, how do i explain this? ive never done anything like this before and it feels so wrong even though in the moment it felt kinda freeing to let go a bit. i guess my question is - am i totally out of line here or is there more nuance to what happened?