Ok I gotta disagree with CWC about the fish looking fine. The belly is too flat and the spine is too arched. In fact that fish looks just like the beginning stages of other platys I've had with fish tb. I hate to be the bearer of bad news, but I've had so many sick livebearers (in fact I've never been able to keep petstore livebearers alive

) I recognize one when I see one. Get her into quarintine & watch your other fish for sinking bellies. Also wear gloves if you put your hands into her tank. If you start treating her now with kanamycin, she might have a chance, though I've never been able to save a livebearer after they start wasting.
Of course it's not possible to give a definitive diagnosis without actually seeing the fish ...but I suspect it might be tb which seems to be common in livebearers. It could also be internal parasites...in that case the fish loses muscle mass due to nutrient deficiency, so it may look like the spine is protruding.