Yeah, yeah, reply nodes suck, etc.
From personal experience and the experience of friends, might I suggest that it isn't necessarily what you think. There are a few other things that might be going on here.
She's just being affectionate in what is, to her, a platonic manner.
Some people are touchy-feely by nature. It isn't necessarily flirting -- she's just demonstrating that you are meaningful to her in some way, ranging from unixy to otherwise. Enjoy it. Being touched is good stuff.
She's being flirtatious because it's a component of her personality.
Some people are flirtatious all the time. They have no intent to do anything more, which is why her having a boyfriend is no problem. They'll flirt with close friends, or people they know, or everyone. It's a part of her personality, and you'll be able to read it better the more you see her.
She's being an evil flirt in order to tease you. In other words, it might be a game.
She sees you as a male chew toy, probably because she figures you'll never do anything in response. Some people like to have a chew toy. If you don't want to be a chew toy, then don't be a doormat.
She's about to break up with the aforementioned boyfriend.
Be afraid. Be very afraid. Problems ranging from vicious ex-boyfriends to being Mr. Rebound may well exist. This is the least likely possibility in my eyes.
In Any Case...
Talk to her. Why are you asking us, when the person who knows is right there?
I'm sure I've missed a possibility or two, or been too general in the eyes of others here. My apologies.