I am sorry you have to go through more testing but if it discovers the cause of your fatigue or any other problems you may be having it is a good thing.
Thallium stress test is a type of nuclear scanning test or myocardial perfusion imaging test. It shows how well blood flows to the heart muscle. A small amount of a radioactive substance called thallium is injected into the bloodstream. You then have a scan. This is a very popular test here in USA.
We want you strong and healthy at that wedding that hopefully is in your future.