Dog scratching and biting herself ...?

Search her skin look for any signs of mites or fleas as these can cause severe scratching and discomfort, if you find signs of either of these there are treatments available at your local pet store. Although if her skin is too raw you may want to seek veterinary advise as some chemicals will harm broken and damaged skin causing further discomfort. If no signs of parasites are found (you may want to try this anyway, as it can't hurt to try both) wash all her bedding and any other materiel she's exposed to often (e.g if she sleeps on your bed or the couch) using an anti allergy powder or soap. If there is no change at all within 1-2 weeks, she will need to be taken down the vets as it could be a medical problem. I wouldn't recommend you use piriton as this is used to medicate other problems and may make things worse. Hope this helps, and good luck I wish her a speedy recovery:)