I see doc. but want to know what causes a fluid filled sac in uterus? is preg. the only reason that would occur or what else n yes was told it's there

Endometrial sac: If you were told its specifically a sac then its usually the gestational sac from pregnancy, though sometimes with a tubal pregnancy (ectopic pregnancy), you sometimes get a pseudosac (false pregnancy sac). Other than that fluid in the uterus may have a sac like appearance. Have your doctor do a couple quantitative pregnancy levels to see what is going on.