Ultrasound question please help I'm really freaking out!!!?

I had an ultrasound today to find out what I''m having, I'm 17 weeks and it's a boy. I knew before the tech. said anything, I have two boys already and know what to look for. This was a one of those places you pay for not a place your doctor sends you. While doing the ultrasound the tech mention how there was a large black mass in the lower abdomen that she has never seen before in other ultrasounds she has done. She also said it could be the bladder but it looked like it was taken up about half the stomach. I started thinking about it and I never have seen anything like this in the other ultrasounds I have had with my other boys. Does anyone know what this could be, I'm going to call my doctor in the morning but help please does anyone have any idea or been through this themselves. Oh yeah and I know that she said it has to be fluid filled.