Why on earth would you put them together?!
I have absolutely no idea what's going on but I suspect there could be a possibility that your goldfish could be infected with a virus or bug in which he or she has no immunity. When you've been isolated most of your life, any contact with another species could bring on an individual based epidemic.
And I suppose a visit to the veterinarian would be financial overkill for a goldfish. You probably just need to keep the bowl clean and wait.
Hopefully someone else here can recognize what's going on.