Hi and welcome to the forum, Denina. Sorry to hear about your platy. Actually, the poo sounds pretty normal for a livebearer. Have you ever seen whiteish-clear spots in the poo? Like in between the red parts?
There's a possibility your new platy may have introduced something to your old platy. For treatment, I would suggest API General Cure, an antiparasite medication that will also help with internal anaerobic bacterial infections as well. You can get this at Petsmart.
What size is the tank and what are the tankmates? If you don't have any tetras, loaches, cories, snails, or shrimp, I would actually suggest treating the entire tank itself. If you have any of the above fish, then it's a good idea to move the platy to a separate tank. If that tank is smaller than 5gals, then you will need to adjust the dosage of the General Cure. To do this, take a container that will hold 5 cups of dechlorinated water. Add 1 packet of General Cure and mix well. Now, the ratio is 1/2 cup of medicated water to 1 cup tank water. So if the tank the platy is in is 2 gallons, you would pour 1 cup of the medicated water into the tank. If the tank is 1 gallon, you'd pour 1/2 cup. You can save the medicated water for 24 hours but then it is best to make a new batch. Follow the dosing instructions for when to dose.