Allergy?

My puppy is now 12 weeks old and has always been itchy especially around the tail area. She often bites just above her tail. She doesn't gave fleas I've taken her to vets to have her anal gland checked and all is okay. She has now clipped the long feathers on her tail so it looks like someone has cut it. I don't think she can be bored, I don't work and spend lots of time with her we have training sessions and nina Ollerton toys. She does get really excited and turns nippy but otherwise when she us relaxed she lies on her back to sleep. I am therefore wondering whether this could be food related. She is fed on symply puppy turkey and rice and has fish 4 dogs superstars as treats. I was wondering if anyone has had any experience of allergies with these products. She is a border collie in case that is relevant . Many thanks 😊