What would a fluid sac in uterus be.but not pregnant

[Mands18]

I was at my Dr's and she sent me to gynea they took smears n swab I them got bloods took that were clear I've had vaginal ultrasound which showed thinking of the womb but a fluid filled sac in my uterus.my hcg levels were at 1 and I've had bleeding and it's been 3 months from I had twin boys by c section that passed.I've had no further explanation and I'm still in pain with some bleeding