Alienated by Co-Workers - AM I An A**Hole or Just Misunderstood?

I spent the better part of a year hanging out with people from my job thinking that they were my friends, only to find out that they aren't (I'm assuming). I had been "seeing" one of them (not technically dating) and she was one of the people I had been closest to but I told her we couldn't be as close anymore because I was interested in someone else. No issue's and she was totally fine about it (at first.) Recently, I started dating the person aforementioned. I really care about this girl and I wanted her to meet my friends. One night, I showed up to a bar where all the coworkers were; my girl had joined me. When my lady made a trip to the restroom, I was approached by a different (female) coworker and asked "why the fuck are you here?" and was accused of making things awkward and basically asked to gtfo. Out of respect for my girl I decided it was best to just leave. The girl who (I thought) was my friend texted me and told me how disrespectful it was to bring my gf around her and to "stay away." Now, I don't think anyone at work was actually my friend - Or they are just better friends with the one girl and have all taken her "side" even though I didn't really do anything to her. The worst part is it's been so long since I've hung out with anyone else that now I basically have zero people to interact with (besides my gf who doesn't really get what their problem is either) So what gives? Does this old friend harbor resentment due to my lack of reciprocating some sort of feelings or did I truly disrespect all of my coworkers by having a gf?