What you are likely smelling is your dogs anal gland fluid. Anal glands (there is one on the left and one on the right) are small pockets on the inside of your dog's anus that contain a foul smelling (often smells metalic or fishy) liquid that usually comes out with every bowel movement. The liquid gives your dog it's own scent....it lets other dogs in the area know who he/she is. Sometimes, dogs can have a problem with their anal glands (usually caused by food allergies) and the fluid builds up in the pockets. When this happens, dogs tend to drag their bum on the floor because their bum is irritated from the build up of anal gland fluid. If this is happening with your dog (may be the reason why you are smelling the smell), you can try adding flakes of bran (the stuff used for baking bran muffins, not the bran flake cereal) to his/her food every day to bulk up the poop which will in turn put more pressure on the anal glands during the bowel movement, and hopefully empty the anal glands with each poop. You can try roughly 1-2 tbsp sprinkled over his/her food daily. If this does not resolve the problem, he/she should be seen by the veterinarian to be sure that there is not an infection, and treat any underlying causes (allergies for example).
Is it a strong metallic smell? Any other symptoms? Any drooling or diarrhea (maybe with blood or mucus), or vomiting or is your pup a little more 'blah' than usual? It could be nasty bacterial infection or two and some other things but the first thing the vet is going to do is run a PV test probably because of the metallic smell, it is associated with parvo. You should get your pup to the vet tonight if there is even the smallest thing different about your pup today. Ate less, drank less, any of the other things mentioned in the slightest. If it's caught soon enough (within the first couple of days) tamiflu can be prescribed in an effort to help but not if they aren't seen early enough. I hope your baby is okay and just has a nasty smelling behind and not any of the serious things a matalic smell indicates.
Or he/she could have an auto immune disorder, my lab died from that...we spend thousands of dollars trying to save him...vet after vet, pills, shots, xrays, and a death later they find out his immune system was attacking his body therefore causing seizures & he bled to death internally. It was the worst pain ever to have to watch him die like that when I was following all the rules from the bet!!!
Go to the vet asap!!!
Go to the vet asap!!!
Diet, whatever he licked last, passing gas. The better question would be, how do you know what it smells like.