So I got a tetanus shot 5 days ago and my arm is hot, red and hard. plus the redness has spread. This is just what I need when I am about to fly in about 12 hrs.
That sounds like a serious infection. You should get that checked ASAP.
Yeah, the tetanus vaccination can be problematic. Take a small icepack with you, or small ziplock baggies that the flight attendant can fill with ice. That should make your arm feel better. Also 2 ibuprofin for the pain. Have a smooth flight!