Twin related question!?

I'm now 22 weeks pregnant with twins. When I was 7 weeks pregnant and had an ultrasound they said their was two placentas. Well then they started to notice that baby a was bigger then baby b. Baby A is 12 oz and Baby B is 7 oz. Baby B also has less fluid around her/him. Now they are saying there is one placenta. Also, They thought twin to twin transfusion but I went and had all those tests done and it was negative still no idea what's going on. Has anyone else had this happen or know someone who had and how the out come was? I'm worried about baby b!!