Cup of Tea and a Chocolate Biscuit

Two nights ago I went to a party where I happened to see a (male) friend I had been at Varsity with. We'd always gotten on famously in any one of the ten-minute conversations we enjoyed during our acquaintance, and this time was no different. While laughing at...something (can't remember, there was vodka involved that night), a face loomed a few inches from his left shoulder - enter the girlfriend.
Now understand two very important things:
1. I have a boyfriend so supremely wonderful, I wouldn't trade him in for all the Christian Louboutins in the world
2. I have never been remotely attracted to this friend (not even the usual obligatory wondering about what it would be like to have sex with him)
Well, either I'm more sensitive than I thought, or she was giving me The Look. You know, the one that says, 'I'm watching you. Keep your grubby paws off my man, ho.' The girlfriend I had taken with me (Le Boyfriend being in another city at the time) assures me there were no such Looks, but I'm not convinced.
The confusion arises from the fact that I am not in the habit of receiving said Looks. I find it hard to believe that any girl would find me threatening in a leading-their-man-astray sort of way. Yes I was wearing a LBD and heels, but so was she - and so were all the models and TV presenters (it was that kind of party).
What gives? What makes a perfectly rational female descend into primal, claws-out jealousy? In general I mean - there was no high-heeled hair-pulling on Wednesday night, just some artificially cheery smalltalk into which I sprinkled Le BF's name like it was salt on popcorn. In retrospect, she probably thinks I'm rather sad...