When Penny spoke to Roger earlier in the night, she was relieved that he seemed to have forgotten about his request for a kiss as a birthday present. If she wasn't convinced that he was actually genuinely flirting with her before, she was after the owl exchange that had taken place a few nights prior. Penelope was having a difficult time discerning how she felt about that specific type of attention after going so long without it. The fact that it was coming from Roger also confused her quite a bit. The last man that had been interested in her had been Percy (well, that she was aware of) and from Percy to Roger was a drastic difference.
One of the benefits to knowing Quidditch player was that they knew other Quidditch players and all night long, Penelope kept spotting various people she knew only from their player stats and excitedly pointed them out to her friends, two of whom were already on different Quidditch teams and seemed to enjoy her surprise and fan-like enthusiasm.
When Roger approached her, Penny had been looking over at a crowd of his fellow Tornadoes. Perhaps she was looking for his familiar face in that very group of men and that is why, when he popped up by her she nearly fell over from slight shock. Her cheeks blushed even deeper than they had been before, due to the alcohol and she put her hand over her heart fleetingly.
"They wandered off," she explained and smiled back at him, doing her best to recover as smoothly as possible. "I didn't think I'd see you for the rest of the night. Are you having a good time?"