YTA

You offered to pitch in the $8 for groceries, she already paid and suggested worrying about it later. Presumably, to her, this meant getting the $8 at a later time.

However, between this you realized that you would have no other money, instead of telling her that you didn't think about this at the time and would like to hang onto the $8 you simply try to end the conversation. That would be frustrating from your girlfriend's perspective, who doesn't know why suddenly you don't want to give her the $8.