Allergies is what I'd think of. Most dogs have mite allergies and they're not easy to deal with. The reason is, is that the mites are pretty much invisible and are in most carpets, even dust particles. And worse, they are in most dry foods - that is the mite excrement and sometimes the eggs.

There are several other symptoms that may or may not be shown including genital licking, swollen genitals, red inflamed ears with or without moisture in them.

We went through a desensitization program with our dog which lasts several weeks and includes injections. It's a process that's never finished, now she only gets one injection a month. We also individually pack her food in air-tght tuperware so that the dog food bag is only only to fill these and is hermetically sealed between openings - this cuts down the probability that our household mites get into the bag.

Having your vet do some allergy tests would be a good idea.