I have had this periodically over the years and have never gone to the doctor for it or even asked.I know it isn't from oily scalp as I have very dry hair and scalp now due to both age and sjogrens.
I will tell you some of the things I have done for it that help off and on. Seems like have to change from one treatment to another. I also wonder if some of it is a neurological thing. Sort of like it itches so you scratch and then it itches more. I wonder if the nerve endings are super sensitive at times.
I have doused my whole head in hair conditioner and wrapped a big towel around my head for several hours. This helped to lubricate my scalp and hair and seemed to settle things down.
I have used a lot of a product called Sea Breeze. This has been on the market for years and is found in the hair products/shampoo department of stores. Not all stores care it though. It has gotten spendy so if you find it, use it and find that it helps you might want to find a cheaper source.
I would suggest asking your hairdresser if she would order it for you. Years ago my MIL(a beautician) ordered a gallon for me and I am still using it. It stings when you apply it but it feels so good. It has several different products in it that are good for killing scalp stuff off.
Then when I am at the end of my rope with the itching I have applied the 1% OTC Hydrocortizone cream to my scalp. Sometimes I have had to rub the cream in all over the back of the head. I will really itch around the bottom of my hair line.
When I scratch I don't use my fingernails, I use the ends of my fingers cause you can rub really hard without the danger of causing open wounds on your scalp.
Another thing that you can try is to take a bowl with a couple of teaspoons of baking soda, add some water and make a semi thin/thick paste. Make it so you can take a cotton ball and dip it in the soda solution and then spread it on your scalp. This is also an old fashioned remedy for mosquito bites.
Sometimes if there is no rash, crust or drainage involved it seems like it doesn't matter what you do, just do something and it will help. Good luck all. Irish