36wks & 5days pregnant i went to the bathroom to urinate&after wiping i had a bit blood on the toilet paper. why is that??

Let your OB know: If you have no urinary tract symptoms of infection or pain it is probably not significant. Sources could be the cervix or uterus. Your ultrasounds would have shown where the placenta is as that can be a source of blood. A urine sample would be reasonable to get. The best advice is inform your OB what is happening.

Bloody show ?: Light spotting can be an entirely 'normal' feature of pregnancy, but it can also be the presenting sign of labor, the so-called "bloody show" that is a presenting sign for the spontaneous rupture of the amniotic sac. Since you are still premature in gestation with your baby, you need to call your OB immediately to be evaluated to see whether you are in preterm labor. Do not hesitate, see your doc

Call your doctor: Vaginal bleeding/spotting can happen for many reasons during pregnancy, including infection, irritation, or even bleeding from your placenta or cervix. At this point in your pregnancy, the spotting could indicate dilation of your cervix but it could also herald concern for placental bleeding. It would be a good idea for you to be examined right away to ensure there isn't something worrisome.

Bloody show?: You may have had a "bloody show", where a small amount of blood-tinged mucus passes near the end of pregnancy. You should report this to your obstetrician or midwife so they can evaluate and guide you properly, especially since you seem to be a little early by dates. Hope that helps.