It could be an abscess, maybe minor or major, depending on the size of the bump. Keep your armpits dry and clean, avoid sweaty places, and consult a surgeon if discomfort persists. You could also use a Magnesium sulphate ointment thrice a day till it develops a pus point and bursts out all the contents, which you could then easily clean with an antiseptic solution like Dettol. Follow hygienic practices and use talcum powder if need be. Avoid anti-Perspirants as they tend to clog pores and stuff infection-causing bacteria inside them producing such infections.