Possibly infectious. Pain in the armpit can be due to some type of infection. You have lymph nodes there that react to infection and other stimuli. You should see your doctor for an exam and diagnosis. If you have a fever, you should not wait to be seen, and should go to an urgent care today.
My left breast near my armpit is sore to touch with lumps. No fever, redness or flu symptoms. Is it mastitis? I breastfeed
Mastitis. It could be mastitis. I would continue breastfeeding and seek evaluation with a physician. If it is an early mastitis, antibiotics will likely be needed.