Why is it when they send my urine out for culture nothing grows yet when my doctor puts me on antibiotics i feel better?

Varies: It depends. You might have bacteria that do not grow in the culture, you might have chlamydia, or you might have some inflammatory conditions that are not bacteria-caused. Most of the antibiotics (if not all) have anti-inflammatory effects, too. This is what makes you feel better. Unfortunately, getting too many antibiotics will also cause resistant bacteria and will make urine sample sterile.

Antibiotics: You may not have a urinary tract infection but an infection in another area of your body. Some antibiotics have other properties such as antinflammatory. Maybe the antinflammatory is the component that is helping you.