What causes the horrible pain associated with a uti?

Probably cystitis.: This is likely due to a cystitis or bladder infection that might have a silent upper tract involvement or pyelonephritis, which is a kidney infection. This can present with flank pain, fever and chills, and dysuria or pain/difficulty on urination. A sexually transmitted pathogen can also cause pain. The cause must be treated with antibiotics, like tmp-smx (trimethoprim-sulfamethoxasole).