I would stay on the same food you are on for right now. I really think it's unlikely to be the cause however maybe checking with your regular vet would be prudent since I can't see your dog over the net.
IF you are going to do a food trial you need to do it correctly and not just change foods to some random brand that advertises grain free or whatever. You need to use a truly hypoallergenic food from a vet for 6-8 weeks minimum. During this time you cannot feed anything else, no treats, no table scraps etc or it will throw off the test. The reason you need to use the vet specific diet is that they are produced so there is absolutely no cross contamination or contact with machinery etc that has touched other ingredients. This food will be expensive because of this, not because your vet wants to rip you off. Honestly I rarely post on vet matters on internet boards because for some reason nearly EVERY discussion turns to food being the cause for any and all ailments in dogs. In reality science doesn't support that. As far as food allergies, studies show that dog food allergies are caused by beef, dairy, wheat, chicken, egg, lamb, soy and then corn in that order.