Melanie,
I assume you've done all the usual things: checked your water, done a partial water change etc.?
Next check the tank environment for hazards - an unguarded heater, jagged rocks, rotten plant or dead snail giving off toxins etc.
If your water is good and the tank is safe, next look at the inhabitants. Have you got any potentially aggressive fish in there? Chinese Algae Eaters (aka "Sucking loaches"), Red-tailed black sharks, tiger barbs, cichlids etc.? Shrimps can be extremely aggressive, surprisingly.
If you have excluded poisoning, accident or violence, the next thing to look for is disease. "Pregnant" with sticking out scales sounds more like dropsy (although they can get like that when in advanced pregnancy, due to fluid retention). The white skin sounds like actual torn skin - you're not going to notice blood due to it being washed away, of course. The orange spots are most likely an ulcer of some kind, which is most likely caused by a bacterial infection. It is possible a bite or injury has got infected. It is also possible that the bite on your dead fish was as a result of cannibalism - to a fish, I'm afraid a dead buddy is just so much free protein (TMI, I know!)
Anyway, as a first step, I always treat the whole tank with Melafix (tea-tree based anti-bacterial, anti-viral, anti-fungal remedy). It won't damage your filter, won't harm the other fish but is amazingly powerful at treating diseases of all kinds. It's good to use as a first step, before you have good symptoms to make a better diagnosis.
If you don't have salt intolerant species in your tank (tetras, otos, snails etc.), a dose of aquarium tonic salts might also be helpful. I believe platties like a bit of salt anyway.
