Today my Dr. told me that my uterus was ballooning what does it mean??

I'm 17 weeks pregnant and had an appointment with my Regular OB. We did an ultrasound for a gender check (I'm having a boy by the way! :)) anyways he found something in my uterus. More like he told me that my uterus was ballooning and he wanted to keep and eye on it because it could cause me to go into labor.. He asked if I felt cramping of any kind and I told him no. He said my cervix was still closed but he put me on bed rest until he can see me again in a week to see if there is any change in the "balloon". I've looked all around online and cant find anything about what he's talking about.. please help! :(