My rats stick to a staple diet of pellets and VitaPrima Rat, Mouse, and Gerbil Mix. Yesterday they got a little tiny bit of yogurt each, and today they got a chopped up baby carrot (which they hardly ate). I took Francis out and he started dirtying the floor- with diarrhea. I cleaned up the floor and put him in the cage, but what could have possibly caused it? Only Francis is having it, and Romeo got yogurt as well. I understand a lot of yogurt is bad for rats, but they only got a lick each. I know it will pass, but I'm the kind of person who likes to know why it happened.

That diet isn't exactly the best. Switching them to better lab blocks like Harlan Teklad (Mainely Rat Rescue and other rescues sell it very cheaply) may help.

Some rats are also lactose intolerant.

Was it watery diarreah, or was it just very soft stool? Has it turned back to normal yet? Make an effort to keep him hydrated, as they can easily become dehydrated with diarreah.