Ultrasound

So, I am considered high risk because of bipolar disorder and because I contracted chicken pox in my first trimester. I've had one ultrasound so far at 20 weeks. I asked my doctor if they were going to try to take another 3d ultrasound at my next appointment because my son's arm was in his face the first time and she laughed and said they probably wouldn't be doing another ultrasound (in general, not just in 3d). They told me a specialist would be coming down from Denver to do the first, and that never happened. I'm worried, and thinking about switching obgyns, but I am not sure if that's a good idea because I'll be 26 weeks on Friday. Any advice?