
I would want to know where the blood was coming from, the uterine cavity is no longer there, Yes some blood in the vagina from during the surgery but should be old blood[dark in colour]by day 2, if it is fresh blood then either the area where the cervix joins the vagina has not been sealed off[sutured/diathermied] or there is bleeding in the pelvic cavity which is finding a way out via the vagina,any collection of bodily fluids can become infected causing peritonitis.
Keep a look out for raised Temp,her wound looking red and any swelling of the abdomen, if seen then back to the vets ASAP.
I hope that none of the above applies to your girl and she is back to normal soon.