What happens during a TB test? I know that you get a shot, but what happens afterwards?

It depends. There are 2 types of tests used to "screen" people for tb. The skin test involves an injection which leaves a small bump under the skin. Your provider will then wait 48 to 72 hours later to "read" the result. The injection may lead to swelling or redness which the provider will interpret & measure. Not all skin changes mean the TB test is positive. A routine blood test for TB is also available.
Skin test. Part of a protein from the TB microorganism is injected into the skin ... If you have had TB in the past, your body will usually react to the protein injected into your skin by causing swelling and redness ... If you ever have a positive TB test, it will usually always be positive afterwards, which then requires a chest x-ray instead of any more skin tests.