I'm having a similar problem with my goldfish. Restocked tank, ich arrived, treated ich with half-doses, had only been at it a few days when there was a big fish die-off. I hate Rid-Ich, it killed two of my favorites last time, too. In any case, my white goldfish has become kind of pink and inflamed-looking and is blood-red around its fins. It also has changed its behavior from pretty chill and foraging for food to swimming constantly and seeming agitated.
Looking it up, it sounds like a water quality issue. Either nitrite or ammonia poisoning. Daily water changes and a filter change were what was recommended. I'd look into getting a water test kit (as I will be doing) or taking a water sample to a fish store to have it tested, just to be certain.