My dog keeps scratching herself all over?

She has a flea and tick collar, and I've been giving her benadryl at least once a day. It looks like she may have a rash or dry skin. She spends her time lying under the deck in the backyard when it is hot. My roommates dog doesn't. Could there be some plants down there she is allergic to? Could she have eczema or some other issue that dries her skin out? What should i do. Is there anti-itching shampoo i could use. Or some otc meds that would help?