One thing might be an ectopic pregnancy. They are a leading cause of maternal death and if it occured very early in pregnancy she may not have known she was pregnant. Miscarriages could also have caused heavy bleeding.
Other causes for a similar set of symptoms are possible, though; some sort of GI bleed, some types of cancer, ovarian cyst bursting, etc. Probably the vagueness was down to they just didn't know what was wrong - no obvious cause, no external wounds, etc.
You might want to search for the woman down the street to see if she was ever recorded as a midwife, nurse, etc. Perhaps she cared for your ancestor before her death.