2.4×2.1mm cyst found on right ovary via CT scan for poss kidney stone (had been having blood in urine and low back pn). Had fluid around it. Serious?

Unlikely. Most ovarian cysts are benign and resolve spontaneously. A follow-up pelvic ultrasound to document stability or resolution is usually the recommendation for incidentally discovered ovarian cysts.
No. . This is normal for your age. Cysts are cyclical in women of child-bearing age and come and go - increase and regress.
Kidney stone. A kidney stone can be very painful and would explain your low back pain and blood in urine. A 2.4cm ovarian cyst is most likely benign but your ob/gyn may suggest having it removed surgically. I doubt it causing your pain. Speak with your doctor.