6 yr old Yellow Lab skin problem

Question
Every summer (only) six year old lab gets extremely irritated skin on entire underbody. Hair comes off, red patches occur.  He becomes lethargic.  Tried antibiotic last year and he vomited after every dose.  Started on cortisone and new antibiotic five days ago.  Have seen vomiting twice, could be much more as he is outside a lot.
Diagnosed as "secondary staph infection" from allergy.  I've been told that "all dogs have staph infections" and this is not a concern.  The day after he started the med he started having a problem with right hind leg, it seems "weak", and takes him a few minutes to be able to walk on it after getting up.  Don't know if this is connected with allergy, medicine or staph infection.  Can you comment?  Thanks for your help.  

Answer
Labs are very prone to seasonal allergies as well as food allergies.  Some antibiotics can cause reactions and because there is a secondary infection, you may need to try different antibiotics to see which one will help without causing vomiting.  There are many other reasons for vomiting, so your vet may need to investigate that.  Unless there is a huge sore on the dog's leg, I would be surprised if the rear leg weakness was related to the skin problem at all.