Hi, may I answer your health queries right now ?  Please type your query here...I found a lump under my right armpit/lymphnode area about five months ago (extremely small). I forgot about it until it got larger last week. I went into prompt care and recieved a ten day antibiotic which has helped some.
Welcome to Chat Doctor. 1. The history suggest most probable cause could be lymph nodal enlargement.2. Lymph nodal enlargement is seen in case of any infection / inflammation, and it can be localized or systemic.3. So treat the cause (any underlying medical conditions).4. If its getting better as you said then its fine wait for few more days until the antibiotic course finish.5. If repeats - Get an FNAC - fine needle aspiration cytology for better evaluation. Good luck. Hope I have answered your query.Anything to ask do not hesitate. Thank you.