I agree you really need to talk to her about this. Maybe there is something bothering her or something she isn't quite understanding. Mabye she thought the other kids would think it was funny or she was clever or something. My son is also in kindergarten, and sometimes I just cannot comprehend the stories he is telling me about things he did or another kid did. I realize he might be embellishing when he tells me, but I don't think he always is.
I really don't think punishment is the answer, and I definitely think one week is way too long at that age. In 2 days she won't make the connection between teh punishment and the act. She might not even make the connection tomorrow. Punishment is rarely effective. What is effective is positive reinforcement and natural consequences. A natural consequence to this act might have been for her to clean it up and miss out on something going on inside with her class. A positive reinforcement might be a reward for consistently asking to use the restroom or one week of "green cards," if that's how your school does it.
I sincerely do not believe that punishment is going to have the desired outcome. I would do what you can to get to the bottom of the "why" she did this. That's going to be your best chance at ending this altogether.