Im 21 and for about 3 weeks my armpit and just under my arm has gone numb it doesnt hurt. it feels strainge when shaveing and feeling it with my hand?

See a doctor: most likely nothing to worry about you are 21 years old you are experiencing what is called neuropathy which mean abnormality in the nerve now the most important issue is what is causing it usually trauma to the nerve caused by example accident that cause injury to the neck or illness like diabetes I know you might not have anything like that so you need to see a doctor to find out why

Pinched Nerve: There is a complex nerve bundle in the axilla (your underarm area) known as the Brachial Plexus. A nerve impingement of some kind could cause some numbness in a specific area or distribution of an affected nerve. This would be best evaluated by a physical exam to determine the specific area affected.

Numb armpit: Numbness can be caused by a pinched nerve, e.g. if you sleep with your armpit draped over the top of a chair, or if tight clothing pinches the nerve. However, there's no way to identify the cause for certain online because the diagnosis largely depends on a physical exam. A doctor should check for unusual masses underneath your arm, and should test the strength and touch in your arm and hands.

Pinched nerve.: It sounds like pressure on a nerve somewhere between your neck and your wrist but that "diagnosis" leads to more questions than answers. Any injury or unusual physical activity? Any weakness in your hand? Do you notice it under other circumstances? Is it getting worse? Think about these questions and make an appointment with your doctor. Together you can get an answer & treatment plan.