What is wrong with my goldfish with large red spot on belly and stringy white feces?

The big red spot on the stomach of your fish is probably an ulcer: bacterial infection that took place after a parasite made a wound there. White feces usually indicates parasites also. I'd recommend treating the tank with a .3% salt concentration, treating the tank with an antiparasitic medication (to kill off the parasites) and start feeding the fish an antibacterial medicated food to kill off the bacteria that are causing the ulcer.