I had a tetanus booster this week, and it is 3 days now. The site is red, swollen and warm to touch?

Warm packs. Apply warm compresses. You may be having a mild allergic reaction. Any time you have had an injection, although very rare, you may develop secondary infection. If this persists or gets worse you may want to check with your doctor.
Normal reaction. After a tetanus booster you can have a reaction which is red, swollen and warm to touch. It can also be a bit painful. This is a sign of how your body is developing a robust immune response to the vaccine. If you start having a pus draining and/or have high fevers, numbness or tingling of the arm/site then you should go back to your doctor.