possible food allergy?

So this is my kitten Shadow. She is 4 months old. i got her at 6 weeks old. Not long after i put her on Blue buffalo wilderness dry food she started having itchy ears. She scratched them bald. I started giving her allergy pills and it seemed better for a few weeks but now shes back at it they are red and itchy. Also has bumps on her head. She does not have fleas or parasites. Only things happening are itchy, red ears and she is always constipated. Any advice would be great. Food suggestions or anything. I stopped her off of her flea spot on to see if thats the cause also.