Can holding in your urine really cause a uti? i thought urine was sterile.

Yes. Good question: Urine is sterile, however bacteria are liable to travel up a female urethra, which is short, from within labia & area around urethral meatus (opening). Regular or frequent urination empies bladder of urine & unattached bacteria. This prevents utis in many women. However, bacteria have time to multiply & attach to bladder lining & cause UTI when subject holds urinating. Specially true after sex.