It's not watery yet, is it?
When we switched out Snickerdoodle's food last year, he ate less hay because he loves his new food. Maybe that could be a factor. As for the watery stools, I really don't know. Has K'ulu come in contact with any other animals? You mentioned that someone in your family has a bird, and birds can carry many diseases; perhaps he/she touched the chinchilla after touching the bird? I'm not sure if you want to try this because I cannot at all vouch for its effectiveness, but I've noticed that Doodle's poops get somewhat harder after he eats some rolled oats. Also, you know the burnt toast remedy for diarrhea (what is the amount? 1 tsp. or tbsp.?), so I'd keep that on hand just in case.
Also, just a suggestion: I've noticed that you are very (and admirably) meticulous with everything regarding your pet, from monitoring his weight, water... everything. Is it possible that you became somewhat overconcerned and that nothing is really wrong? I do not mean to sound rude, and as I mentioned before, I admire how thorough you are. Are you sure that something is different?
Best of luck to you and K'ulu