client asking modification on her paid and completed order! shall i ask for a new payment or not!?

hey! so i have this client whom I have submitted her order and followed the instructions and details she gave. She had completed the order and gave me positive review for it, so I was happy thinking there will be no problem anymore since I followed precisely the details and she said she liked it! the next day she wants modfication, saying she wants it to be modified! shall i ask for payment?
i'm afraid she might already be using those files I gave her and, I, will be doing a new one and she'll be using it again for FREE!