What is wrong with my bunny?

We got our bunny last week around thursday. Im pretty sure its young. Lately, he/she (dont know the gender) has been having diarreah pretty often. We are forced to clean it in a sink (but we dont use alot of water and only the lower half). Now it seems to be seccluded and less playfull. Note: we feed it rabbit food from the store, yogurt drops(only as treats), it nibbles on the wood toys and it ate some grass one time we took it outside