I'd act like nothing happened. Granted I don't know snapchat that well, but it seems to me the more you take pains to make everything "seem cool" and like it was no big deal the more of a big deal it's going to seem. So don't make it awkward and don't explain. If you like this girl so much, though, it might be worth considering telling her how you feel. It's not fair to yourself to live life analyzing every microscopic interaction, especially ones on apps or online. Don't you want to know?