I recommend immediate veterinary evaluation; diarrhea, fever and not eating can be symptoms of parvovirus infection. Other possibilities include bacterial or parasitic intestinal infection. The veterinarian can check your puppy's stool for parvo and parasites, and based on testing and physical exam results they will be able to prescribe appropriate therapy.
Kimberly Coyner, DVM
Dr. Coyner is most correct...you need to get your pup into the veterinarian right away. As she said, parasites are a potential problem, as is a bacterial infection, but with a rottweiler puppy at 5 months old, parvo jumps to the top of the list. It seems that Rottweilers (at least in my area) have a propensity for parvo. What is his vaccine history? (How many, when were they given).
Also, puppies potentially get into trouble by chewing and swallowing things that they shouldn't.
I wouldn't delay...get him to your veterinarian today.