My dog has a chronic itchy skin condition?

Our dog has been having an itchy skin condition on and off for the past year or so. We believe this comes from some sort of allergic reaction. We are not sure if it is dietary, environmental, or a combination of both. We occasionally feed her a little cheese and popcorn, which she loves. We also think maybe it could have something to do with the carpet shampoo??

She seems to be itchy all over, but mainly on her front flank.

Have any of you had your dog experience skin allergies? What was it from? What did you do to resolve it??? I thank you, and Shelby (my dog) thanks you, too! :)