Is it bad to take antibiotics if you are not entirely sure you have an infection?

Yes: It's a risk-benefit thing. But, moreover one can never be entirely sure if they have a bacterial infection, that is, before the cultures and/or blood tests, the epidemiology and the disease signs, symptoms and progression are known.

Antibiotics: Antibiotics are reserved for treating bacterial spirochetal and mycoplasmal infections. If your diagnosis is not one of those conditions, there is no reason to take one. Taking an antibiotic because feel you have an infection will only help bacteria develop resistance making bacterial infections more difficult to treat. It is getting difficult to create new antibiotics as it is.