Could Blood In Urine And Burning After Urination Post-sex Be A Bladder Infection?

My girlfriend is having issues,for the past 3 days whenever she uses the restroom to pee,there s blood in here urine and she seems to think that it s a bladder infection.and if it is a bladder infection how did she get it?me and her recently had sex and afterwards it burned when I urinated and that hasn t happened before and she says she hasn t slept with anybody else.is she lying