After basic blood work my doc wants to get sample of fecal matter and pelvic ultra sound. What would cause her to want this because of bloodwork?

Triage testing. If by basic bloodwork you mean a CBC and a chemistry panel, then the most likely reason for concern is anemia. The common sources of blood loss leading to anemia in a young person would be intestinal blood loss or menstrual losses. Fecal sample to look for evidence of occult blood and US to assess uterine structures seem reasonable.