Hm, I 'd have asked that first. If you had used a new soap or detergent as that does sound a bit like allergic dermatitis. A lot of the time, itching behind the knees and hips could be because of atopic dermatitis. Otherwise known as ezcema. Your itching the area causes the skin to be funky after. Do your best not to itch. I use a balm bought on amazon but also sold in drug stores for itchy ezcema skin. Aveeno Itching releif balm. Works great. This is info on ezcema. https://www.mayoclinic.org/diseases-conditions/atopic-dermatitis-eczema/symptoms-causes/syc-20353273