My Girlfriend Is Having Issues,for The Past 3 Days Whenever

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