Bald spot coming back on my dog after four years?

Hello DS, I'm aware of what time it is, I just need to make this quick.

Earlier tonight I was brushing my dog with an average brush then a flea comb. After a little over half an hour I had a pile of shed hair bigger then my hand. That's normal for her, she's a beagle/Italian greyhound mix, she sheds a lot every year. What concerned me was that it had numerous skin flakes in it. She has dandruff, but it hasn't gotten this bad.

Four years ago she had an allergic reaction to something, and got a HUGE bold spot on her thigh, which went away after we applied something (I don't remember what) the vet gave us. Today I was looking at her and it's coming back. I thought it was just the food, (back then it was Gravy Train.. -.-) until it came back today. Now she gets TOTW Wetlands formula and we alternate between that and the High Prairie.

http://tasteofthewildpetfood.com/products/dogs/dry...

http://tasteofthewildpetfood.com/products/dogs/dry...

She's only allergic to grains (not poultry or beef or any meat), except for rice and she never even gets that. And my other dog gets the exact same thing and his skin is perfect, and they have about the same allergies.

We're taking her to the vet Wednesday, and we're taking the handful of fur I combed so they can have a look, but I was wondering if anyone may know what it was already or they've had something reoccurring like this before.

Thanks!