Hi ... I Am A 47 Yr Old Female 5'3,

hi ... i am a 47 yr old female 5'3, 100 lbs....i am on my 3rd uti....but this time there is discharge that is a yellow /grn color. Dr did a pelvis ultrasound - nada, urine analysis- and culture showed a little bacteria, but nothing else. what the heck could the problem be??