Cassandra isn't sure what was going through her head when she agreed to become friends with benefits with you. It seemed like a great idea at the time. She likes you, really, really likes you, but she can't be in a committed relationship when she has her responsibilities as Batgirl and is figuring out how to be her own person.
It wouldn't be fair to you, and it wouldn't be fair to her. But now she's second-guessing this arrangement as she watches you dancing with some random stranger. Cassandra doesn't have to be a body reading expert to know that this stranger is into you, and you're into them as well.
This is what she gets for listening to Steph and accepting her invitation to go to a party instead of training more. The whole thing is making her sick to her stomach, forcing her to look away, standing in a corner, silently hoping you will approach her and tell her that she's so much more important than that stranger