Mickey Mouse Platy Help!!?

I have three MM platy's and one happens to have a red dot/spot on its side,since I got it from the store. As of lately its started laying down on my plants and hiding in my decor. what could this mean? IDK if they are male or female and this 1doesnt seem to be bigger (doesnt look pregnant) and it moves around and swims whenever Im around. I do have another platy that has grown GREATLY almost to the point where it might be pregnant but it doesnt have the black spot (a dark shading but nothing more I dont think) on the back of its stomach. Could it be pregnant and this other platy (possibly male) be its mate and is taking care of an area of the tank thats for the birth? or could my fish just be bloated and the other be bummed out/ possibly beginning to die? I intend on changing the water this weekend and has been circulating for about a week and a half but the water doesnt appear to be an issue. I have1 red eye tetra (thats a little agressive history),5 glo/neon tetras, 3 MMplaty, 2frog