Marinette had definitely not come to the party alone. First and foremost, she came with her friend. Who she also had the biggest crush on, though she never fessed up about that. Problem is she ate so many breath mints before coming out, that her stomach was a little queasy. The fact that she was on a date - a real, actual DATE, with Adrien made that a LOT queasy. But she refused to let that ruin the night! She just needed something to settle her stomach, so after a short while she had casually excused herself and wandered over to get a (non alcoholic) drink at the bar.

... she could barely hold the cup. She was so focused on not spilling it on herself, she didn't notice anyone else. Careful not to startle her, she might spill her drink on you!