Beignet: Itching, Scratching,Open Sores, Vets,Shots, and medication. Flea allergies! Oh My!

For about a week now, my poor Beignet has been itching and scratching with no relief. She has always had really bad skin problems but never to this point; open sores. She has a suspected chicken allergy so we have her on Poultry free dog food that we coat in coconut oil and fish oil. She is also on a daily antihistamine and she was on a flea preventative but I had lost track of it and hadn't been keeping up with it.

The other day I did find 1 flea on her and didn't find anymore so I figured it was probably just a "straggler," and nothing to worry about. Boy was I wrong!

I noticed she was scratching more than normal and I would go brush her/comb her to try to offer so relief but as soon as we came back in the house, her itching and biting would resume. Then yesterday, I noticed an open sore near the top of her tail bone.

Today, we took her to the vet; According to the vet, "dogs like her," (I am guessing with skin allergies issues) "one flea bite is all it takes for contact dermatitis and sores."

Long story short, Beignet was put back on her Bravecto (last 3 months)- which if I had stayed on top of it, would've only cost me about $50-$60 tops, but now because I had let it slip by the waste side, A normal $50-$60 ended up costing me $113.88 because she also now needed a shot plus antibiotics.
Why do I always have to learn lessons the hard way? LMAO. Anyway, baby girl has gotten so much relief and is currently sleeping peacefully (even though it is raining outside and she normally hides in rainy weather; that shot must've knocked her out!)