I have an itchy armpit and when I scratch it, it hurts real bad why is this happening?

Skin irritation. The armpit is itchy. Is there a rash? That area is very sensitive. If it's irritated, it will be itchy and a cortisone cream can clear that up. Is there a lump in that area? If you have a cyst there, it can be itchy, and tender so when you scratch it, it will be very painful. Try the cortisone twice a day for a week. If it persists, see your dermatologist.