Why is my fish fat is it sick ?

It could be shes a prego or it cold be it has bloat. Bloat results from when you feed a fish too high of a protein diet. I would just wait and see but if it stays in a plant too long I would ask for some help from someone in your area that knows about fish. Even with the Video its hard for me to tell whats going on without being there myself. Hope all goes well and good luck.