Under the circumstances, instead of refusing to get a drink, I would have established a quid pro quo.
Her: Hey, can you get me a drink?
You: Sure, but what are you going to do for me when I get back? I could sure use a backrub
Her: option 1 - ew, no, I will never touch you, I am repulsed by you.
or option 2 - Hmmm, we'll have to see.
or option 3 - ok, sounds fair to me.
If 1, don't get the drink. If 3, do get the drink. If 2, your call.
Also, just thinking outside the box here, but you could have said, "sure, I'll get you a drink" then wander off and talk to another girl within eyeshot of the first girl. Play the same game on her she's playing on you. If she comes over asking about her drink while you're talking to girl 2, that's good stuff.
CAT FIGHT!
There are no hard and fast rules, stay flexible and try not to focus on just one girl or she will be able to play games like this on you. In a party situation, keep a few open loops with a few girls so you can bounce around.
Good luck.