Very itchy skin of a sudden; what's the cause?

A few months ago I went to the hairdresser, and immediately after leaving my scalp itched a lot. I figured it was because of the products she had used and thought it would go away with time. But it didn't go away even after several weeks. So I checked all the obvious causes (eg. lice) and didn't find anything. I would have defined the itching as "annoying" but nothing too serious.

Recently (since about 1-2 weeks), I now itch EVERYWHERE. Scalp, arms, legs, hands, feet, etc. I don't know if it has anything to do with the hairdresser thing or just coincidence. Visually there seems to be nothing special about the places that itch. But I've noticed these past few weeks that my skin feels a lot more dry and sensitive than usual.

What could be causing this?