Itching is a common condition. The sensation of crawling underneath the skin is also well described, although somewhat more rare. Medically this is known as formication. This requires medical attention. I would recommend that you see your doctor
Formication can be caused by many conditions. The classic condition that causes this is both the intoxication and withdrawal from cocaine. Other synthetic drugs like methamphetamines or stimulants (like those for ADHD) can mimic this. If any of these are applicable to you (or if you recently started any medicine / illicit drug) I would STRONGLY recommend seeing your doctor. Alcohol withdrawal can also cause formication. This could be a sign of sever alcohol withdrawal which can be fatal and therefore requires medical attention. Again, talk to your doctor immediately if this applies to you.
There can be other causes. Often this can be caused by a neuropathy, or problem with the nerves. Nutritional deficiencies, Lyme disease, thyroid problems or syphilis infection are common causes of nerve problems. Again, speak to your doctor.
There are skin problems which can also cause this situation. Dermatitis, or inflammation of the skin, can be caused by many things but may be interpreted as formication. Again, see your doctor.