What does white blood cells in urine mean?

Was at myOBGYN today for a pap & to check on a cyst and I gave a urine sample & she said she's gonna send it to the lab for further testing cause there were white blood cells in it. I'm trying not to worry but what could be the cause of this? Anyone have this issue? I've been having painful sex & bleeding & back pain that's why I went to get checked