If you have started a relatively new prescription, that can be an indication of allergy.
Some people itch from almost anything and you don't have to have a rash. I would guess you might want to check the soap you use to wash clothes and your body soap, even perfumes or deodorants. To try and say what is causing it would be like trying to find a needle in a haystack so the best way to maybe narrow it down would be to think of what you might have changed recently and try taking that away ... even hand soap can do it. Not a doctor so this isn't medical advice just a way to maybe pin point what happened to make the itching start.
Sometimes if you exercise (or even move around briskly) and are a little dehydrated the water that's trying to squeeze out of your pores, can get caught up on the oil on the pores, and make your skin feels itchy.
You should also check if you switched detergents or shampoos or soaps, as cheaper shampoos and detergents have ingredients that can irritate the skin to the point of where you feel like you have a rash, but your skin is just itchy.
You could also be allergic to things you submerge yourself in besides shampoo - like pool water - if you have a bleach allergy.
Remember too that allergies can appear later in life that were not there when you were younger.