Why is my dog scratching all the time?

I have a little rat terrier female fixed. When i got her last rabbi-es shot the vet gave me some medication for her to give in peanut er saying that probably wheat like pizza crust etc are making her allergic. We have not given her any table scraps and she is raw by her legs and where her coll er is? I have been putting bacitracian on it but, feel aw full If I even go to the vet It is $100 .