i do know when we got our two rabbits from the rspca (we've had them for about 6 years now) they gave us a list of what rabbits cannot eat and LETTUCE was one of them, also if you let them run on the grass dont let them eat dandelions. I do know that too many green vegetables gives rabbits diahorea and apparently diahorea is fatel to rabbits, if you dont cure it quick it kills them quite fast. You can look up on the internet what is and isn't allowed as food, you'll be suprised what they cannot eat. However it may not be food related in this case but keep a close eye on the other one and dont clean the cage every day, maybee 2 or 3 times a week, they are supposed to eat their own poo to aid their digestion. But if you've only had it a day its probably nothing you have done, probably just been unlucky and bought it already sick. So i would go back to the store.