Why did this just happen?

I'd do a 25% water change each day for 4 days, give the filter a clean (in aquarium water of course), dose with something like stresszyme and see how it goes. As for the sickness it's hard to tell but pimafix isn't going to kill internal bacteria, it's an antifungal so it won't help, you're probably going to have to get a different product that will work internally.