It Has Been 7 Months Since My Baby Was Born

It has been 7 months since my baby was born and my uterus has still not contracted to it's normal size. I went to see a specialist and they noticed a small mass in my uterus. I had a complicated delivery which involved a D&C of my placenta right after delivery. Could this be a remnant of the placenta that is keeping my uterus a large size? I have an appointment for a Fluid Ultrasound in the coming weeks but would like to get an opinion on this.