I wondered how their could be TWO chis with the same problem!
It really could be food allergies...if she is absolutely fine in every way, start with a food switch. The thing is, mange can get away from you pretty quickly and then you have problems, so I would probably (if it were my dog) go back to the vet for a skin scraping. Did the vet notice hair loss when you were there for the lump?
When you switch foods, you might start with California Naturals (by naturapet). Google naturapet and you'll find all the good stuff about that food. We used it when Dolly had mange. We now feed the dogs Innova adult and they love it, have beautiful coats, etc...
There are lots of good foods for allergic dogs. Foxy is right...corn is often the culprit!