My fish has black on her tail?

Ok so heres the thing. Recently my fish had fin rot and fungus which i treated and she got better. I went away for 3 days whilst my parents where feeding her and whn i got back she had a black out line on her tail. She is all orange (a goldfish) but the tail which was most affected by fin rot has now got a black outline. Its really wierd. My other goldie is completely fine and has never been sick before. Can i mke her better or do anything? It hasnt been getting better or worse and hse is still eating and being really energetic she doesnt look sick at all!