Any other symptoms besides that? Mucus discharge with the poo or any thing like that? Is the fish eating? Are any of the other fish showing signs of this as well?
What is their diet?
For future reference, feeding peas every so often (I go about every week) will help their digestive systems. Use frozen (not canned - too much salt) peas and just defrost them, squeeze them out of the outer "casing" and mush between your fingers. Deposit into your tank. The fish absolutely love the stuff and it is good for keeping things... moving. This is a nice preventative against constipation, which is one possible cause for the strange behavior.
According to what I've read, this medication is safe for all fish, but you have to take that with a grain of salt. I am not a vet, nor did I stay at a Holiday Inn Express last night. I am by no means an expert, but this is what my research has found. These medications should be safe for your other fish, but I guess there are no guarantees on that. What other fish do you have? Are there any that have known sensitivities, like corydoras? Is your temp at a steady 72, with occasional swings from 68 to 74, or is this a daily occurrence? Temp swings like that can be stressful, which can leave the fish more susceptible to issues. If your temp isn't holding at a steady temp with only a degree swing, I would say that you need to get that sorted out as well. Does the platy hang out near the heater when the water temp is colder in an effort to warm up, or is it just trying to hide somewhere?