It is quite common for the physical stress of an infection like a UTI to temporarily delay your period. Since your pregnancy test is negative, illness is the most likely cause, though you can retest in a few days to be sure. UTIs are bacterial and require prescription generic antibiotics, such as nitrofurantoin or sulfamethoxazole/trimethoprim, to clear safely. Please consult a doctor for a quick urine test to get the correct prescription. In the meantime, drink plenty of water to help flush out the bacteria. Please do not worry, this is very common and easily treatable.