The water at my house can leave a pink "film" behind if allowed to sit in a dish for too long (I only deal with it when I've been out of town for a week and come back to clean the critters).

Is it possible you cleaned a bowl and there was a bit dislodged and still in the dish when you re-filled the water dish?

If it's not - then my next concern would be blood from somewhere - if it's from mucus from the mouth, I'd be heading to a vet - as that's highly irregular!