I have pink spots in one armpit that are non itchy but burn and are sensitive to touch. could this be from an under armor shirt irritating this area?

Low grade infection: You may a low grade infection from shaving under your arms. Try applying an antibiotic ointment for relief.

Possible yeast: Irritation from a shirt would likely affect both armpits, as well as other areas, and would typically itch. This is more likely to be a yeast or fungus (athlete's foot-type) condition, which is common in warm, damp areas such as armpits. Try an otc antifungal cream twice a day. If there is no response, you'll need to see a dermatologist for a direct examination.