i have a red spot that itch under my armpit is like a rash
The underarm is a nice warm, dark place for all kinds of organisms to grow. Add to that possible allergies to deodorant and irritation from shaving, and you have many possibilities for what could be causing your rash. Cancer is probably the least likely possibility. You can read more about some of the kinds of rashes common in the breast area at this post on Rash or IBC. Be sure that you are drying the area well and use a cleanser designed for sensitive skin. If the rash gets worse or if it doesn't get better in a few days, consult with a doctor. Putting the wrong kind of ointment on a rash can make it worse.