It is called formications and there are several different things that cause it, including illnesses, hormones, fevers, and certain medications and drugs.
If you Google "formications" you'll find a number of websites (wikipedia, et.al.) on it and you can see if any of the causes are a possibility for you. Most of them are treatable.
Definitely I have a case of birdmites. Struggling with this for 6 months now. Financially drained, unemployed & seriously, seriously about to live on the streets. Threw out all furniture from one home last November & now ready to just walk away from present home as all my furniture & clothes are infested again. Unable to get any decent sleep & my car is also infested inside. No money & don't have any way out of this problem.