you haven't adapted yet. give it some time and it will become more natural and fluid. when you feel you wanna stop, keep pedaling cuz that's all you can do on a fixed. it took me about a month before I really began to get comfy on it. common sense will tell you that you won't have to adapt back to your mtb because you can coast. btw what's your bike/set up and how long have you been riding fixed?

read this as well to help you transition:
FG 101