Dog scratching, itching and biting herself

My dog keeps scratching, itching and biting herself. There is no signs of fleas, and she has been given a flea treatment recently. Could it be hot spots? There is no sign of blood or scabs. Only red marks where she has scratched. She is a Saint Bernard. Is there something I can try at a pet store? I use oatmeal shampoo..

You must take your dog to the vet - there are so many things that can cause this.

My dog also used to do this every August - it was seasonal allergies in his case.