Ivermectin is excellent for killing intestinal parasites (though not tapeworm), and it is fairly effective against tick infestations, although it takes several days to kill them. It also prevents heartworm, which is endemic here. However, it is not useful for blood parasites like ehrlichiosis.
Having said that, you say your dog was treated with two injections plus antibiotics, so I suspect it was babesiosis. There was a laboratory-based study a couple of years ago suggesting ivermectin had an inhibitory effect on Babesia. I can't remember the exact details, but the results were promising, so you might be seeing the effects of that.
(Note for anyone else reading this, do NOT start giving your dog ivermectin without getting them tested for heartworm first. If they already have a mature heartworm infection, it can kill them. Once you know they are clear, it must be given every month as a preventative. If you don't give it for more than 8 weeks, they need to be tested again before you restart.)