Why was I asked if I was taking antibiotics during my chest x ray
You might have been asked if you use antibiotics because its traces might be high in your body but it is not working. A recent study states that antibiotic commonly prescribed chest infections do not work.
Probably just a routine question. (Or if they are checking you for tuberculosis maybe they need to know.)
if you were, they might tell you to finish and come back because it might affect the x-ray results.