How do I stop my 10 month old English Mastiff from chewing herself to mutilation?

Is there a good skin conditioner to use that might help? She really did a number on her hind quarters a couple weeks ago, and the vet bill was not cheap. She's almost completely healed now, but still has to wear her E-collar, or she tries to chew those places again. I changed her food to something that has no grain, thinking she might be allergic (dogs don't "graze" anyway, so I don't see why you'd feed one something a cow would eat.) And the vet put her on flea prevention even though I have never ever seen a flea! Any advice on a skin conditioner or something will be appreciated! Thanks!