Wow, you bought her on Sunday and tried to feed her that night?

It sounds like your setup is okay, but you will want to give your snake a period to adjust to her new environment.... usually 1-2 weeks. If you bought a medium sized rat it sounds like you bought a decent sized Ball. Sometimes the older ones need a little longer to adjust to their new environment, so don't be surprised if your snake doesn't eat for a few weeks, especially if she had been eating medium sized rats before.

Give her time to settle in and don't handle her until she starts to eat. If her environment is comfortable for her, she'll eat eventually.

Good luck!