Tetanus booster is basically inactivated toxin administered. It produces delayed hypersensitivity reaction.
It does have a side effect, like any other toxin if injected into your body. Due to the delayed hypersensitivity reaction there are a no of problems occurring like fever, redness, swelling around the injection site. But what you have developed is absolutely normal a reaction to the booster.
These toxins basically induce production of antibodies, which will protect against tetanus. You can wait for another week and see if it disappears. If it doesn't then I suggest you visit a physician.