It might be fleas or ticks. She could have something irritating her skin. If I was you I would put some tick flea shampoo on her and if that does not help then just taker her to a vet to check her skin.
Alot of dogs and cats have alergies just like people. It could be due to the food. Alot of dog foods have food dyes. They could also have a beef allergy. Also depending on where you live it could be mold, or different types of trees or weeds blooming out that cause an allergic reaction. One flea bite can set up a reaction. There are lots of things causing it. Your vet can give the dog an injection to relieve the itching and depending on how bad the skin is you may need a course of antibiotics.
Use a flea comb to check for fleas. If you have ticks in your area, check for them also. If it's not either of those, move on to other possible causes. If she rolled in something different recently, she could be having an allergic reaction. If you changed her food recently, same thing. My dogs don't seem to be itchy just from shedding, but if you're not grooming her and getting rid of the shed fur, she could develop mats, which will irritate the skin. Some dogs just need to be bathed more often than others. That might help; oatmeal-based dog shampoos soothe itchy skin. (No human shampoo for her.) Fish oil capsules can do a lot to keep the skin moisturized and healthy. Whatever's going on, it's good to figure it out. Too much scratching can lead to red, irritated skin, even open and weeping sores. Then it's a series of trips to the vet and antibiotics and hefty bills. Take it from someone who's been there, you don't want to go down that road!