Sometimes when they go to the bathroom some of them tend to walk while rubbing their tale to get the rest off, and while doing that they can rub it right back on to them and not no it. After a few days it tends to smell so a bath may be in order, they will puff up and usually not tend to like a bath at all. Make sure temps are close and just wipe like a little baby their butts and you should be good to go. Now you can't bathe them to often. Also when done bathing make sure that their tails are completely dry as they can tend to get tail rot pretty easily.
It could be among many other reasons as well but that is one noone touched on so i did, but i do agree with some other suggestions like parasites, runny stools, and so on.
Good luck