Your Bun could be having digestive problems, sometimes their poop does change every now and then, depending on what they have just eaten, but rabbits' poop should mostly and regularly stay a uniform size, shape, and texture if healthy. I would guess maybe it is having digestive problems, has your Bun been drinking enough water?
Some other people will probably want to know if your Bun has been acting differently, so you might want to give us some more info on whether your Bun's activity or eating has changed
Someone with more knowledge might be able to give you some better ideas