Are any other fish in the same aquarium showing signs of illness?
Always test the water quality first. If this is fine and it is just the one fish, has she given birth recently? Swordtails, like guppies, tend to be pregnant non-stop and this causes strain on the females. Try and separate her to give her a rest. I would float a container with a bit of plant added in my main tank and clip it to the side with a peg if I wanted to keep it in place. Clear plastic is good but bear in mind, if you use a white, or light coloured container (like a 2L ice cream container) the fish will try and match its surroundings and turn very pale. This water will need changing daily, using water from the main aquarium, to keep it fresh. Add a bit of salt (about 1 level teaspoon per 4 litres).
Apart from the spot and getting skinny, are there any other signs of illness? Does it stay at the top, or bottom, of the tank? Are the fins clamped? Are the fins ragged or split? Some of these could indicate a bacterial infection or parasite but if it is just one fish and the rest are fine, I think it more likely to be damage from birthing.
Do you have any idea how old the fish is? Remember these fish only live 5 years max (often only 3 - 4yrs)