I have severe itchy spots on my arms, fingers, back of my knees/hands & ankles. After scratching I get white elevated nickle-sized spots. What is it?

Most likely.... Dermatitis in these areas is most likely an allergic reaction, systemic. Perhaps a food. Early eczema or even psoriasis could also be the culprit. Please visit your dermatologist for a definitive diagnosis and proper treatment.
Dermatitis . Sounds like contact dermatitis which is allergic relation to a new substance like laundry detergent, fabrics soaps and the like and suggest you see a dermatologist.
See a dermatologist. It is impossible to tell without being able to see the leasions. See a dermatologist.