Does your uterus shed blood cells before your menustral starts is that why i had small trace of blood in urine. no uti or anything else.

Urine test: If you test a urine specimen, it is very sensitive for any blood. You could easily test positive due to menstruation before your period is apparent.

It depends: Menstruation does not just turn on like a faucet. So there maybe a little bit of menstrual bleeding just before the full menstrual cycle begins. The small trace of blood could also be from small tears in the vaginal walls after intercourse. Or the blood might be coming from the bladder, or kidneys due to infections. So it all depends. If you have painful urination or burning than see a doc.