A uti (urinary tract infection) will not affect the test.
Girls only (usually) have a period once a month.
Make sure you take precautions when having sex ie use a condom.
If you have been having unprotected sex, then I suggest you get her to see her doctor - a negative test could be because a) you tested to early, b) she hasn't produced enough hcg (pregnancy hormone), c) not a sensitive enough test, there maybe other reasons.
But get her to check with her docotor and make sure you always use a condom. (as they also protect against infections unlike birth control pills)
if you can, get condoms that contain "nonoxyl-9" this is a spermicide or use a *water based* lube containing nonoxyl-9.
I say water based as oil based lube's can perish the rubber/latex of condoms and render them useless