As far as over-the-counter medicines go for dogs, many times they are not safe. They are formulated for humans, and what is ok for humans is not always safe for dogs, and sometimes even deadly.
However Benadryl is fine for dogs. If you gave him Benadryl that's OK but the dosage has to be right.
The dosage is 1 mg per pound of the dog's weight.
So you would have to see how many milligrams there are in 1 tablet, and break the tablet accordingly.
It sounds like because he is a little dog (how much does he weigh?...10lbs?) You gave him too much. That is why he was sick no doubt. The best answer is to ask the vet's advice about this. You could do this over the phone perhaps. I would not feed him for a while, let his system rest, but make sure he has plenty of water. But please do speak to your vet because I do not know what effect too much Benadryl could have. You need to check this quickly.
Oh, the immune system can be failing for other reasons. I just know what the cause was for my dog. It might be worth an ultrasound and blood test just to check things out a bit more. Would have been nice to know your dogs age.
At the risk of upsetting some people, I had a dog once with that same itchiness and bad odor. It is the immune system failing and if not handled properly, things will get worse. The cause of these in many sensitive purebreds are vaccinations. I stopped vaccinating my dog. Then I did everything I could to boost the immune system. I starting cooking for her as opposed to feeding dry or canned food. Raw food diet is also good. Do research on these things and find a good balanced diet. If you can afford it, there are some natural diets that can be delivered to your home frozen. The point is, no processed foods. Zero. If this is what is happening to your dog, the vet can't help you. Stop giving all medications. Find a vet that does acupuncture on animals. Also, there are great holistic vets out there that can be of a huge help to you for this type of problem. The traditional vet will only treat symptoms and the problems will get worse..........this is from my personal experience. Good luck. I would avoid any cortisone as it would hurt the immune system even more. An oatmeal bath might be temporarily soothing. HOLISTIC VET for this one.
Did you talk to your vet first about giving him over counter meds, there are some over the counter meds that are safe for dogs but you should have asked you vet first benadryl claritin and zyrtec are safe but a vet could help and tell you what is safe dose for your dog.What is causing the skin allergies fleas, food, to help him in the long run what never is causing the allergy needs to be addressed and treated.Here are some suggestions to help what going on now this has helped us in the past but pls call you vet first and make sure it OK for your dog.To help with the itching give him a cool bath in a oatmeal base shampoo also 100% aloe helps itching or essential oil of cedar wood and lavender helps cool the itch also hydrocortisone cream helps inflammation and itch also keep the skin dry to help vomiting pepto-bismol 1/2 teaspoon per 10 lbs every 4-6 hr for 24 hr in till he stops vomiting but call vet to be sure it ok.Hope he better soon.I would also call and tell him what you gave him and see what is wrong.