Hi @nvmoen and welcome to Mayo Clinic Connect. I'm tagging fellow members who have experience with small cell carcinoma of the bladder, like @tattrigoo @samasama @jimmy2248 @sandielegal @marycedroni @whitneyvet and @predictable. You can read more of their posts in this discussion:

- Bladder Cancer/Small cell bladder cancer https://connect.mayoclinic.org/discussion/small-cell-bladder-cancer/

While the PET scan showed no metabolic activity of the tumor and no new tumors (great news), I can understand that you're concerned about this recent apprearance of blood in the urine (hematuria). Mayo Clinic listed quite a few possible causes here:
https://www.mayoclinic.org/diseases-conditions/blood-in-urine/symptoms-causes/syc-20353432
Have you had an appointment with your oncologist since the hematuria incident? Any chance you may have a bladder or kidney infection?