First, if you would assume this is a sick fish, I would suggest to put this in the "Unhealthy Fish" section to get more concrete and specific answers.
Second, based on your explanation, and the fact that there are other guppy's, I would add to the theory that this guppy might be pregnant. Search for a Gravid Spot on the back side of it's stomach. If there are male and female guppies in this tank, the chances that it is pregnant are almost 100%. If you can, try again with a different or the same camera and try to get a pic that is slightly more focused. But from what I assume, I would say it is pregnant.