Tetanus shot the day after the incident?

It is possible that researching tetanus has made you get paranoid about it, but all the same, muscle spasms and lockjaw (when you can't open your mouth at all) are signs of tetanus so you should get checked out. Though you have already been to the ER, that doesn't mean you're automatically okay. Tetanus shots, I've heard, take 4-5 days to work. Cuts and scratches don't tend to be as bad as puncture wounds though, so don't stress too much. Tetanus shots are meant to prevent tetanus. If the bacteria is already in the body, then there's no actual guarantee. You should really have a tetanus booster every 10 years.