Hello,
I have no age and sex of yours which would be a very relevant info for answering your question. Having said that I would give you a basic idea of your condition.
Such lumps are more common among teenage women particularly, however, men do get them at times. As you said that its not a hive or a bite, I would consider two possibilities:
1) Apocrine gland blockage - Generally the glands are present in different locations in your body including the armpits. It starts to secrete substances after
puberty and sometimes due to cosmetics, perfumes, under arm roll ones the glands gets blocked and it becomes super infected by the microbes normally residing in our skin.
2) It can be an axillary
lymph node enlargement - This can generally happen after a
vaccination or so as these are the sites which mounts an immune reaction in the beginning. In both cases I would suggest to wait and watch for a day or two if its subsiding. If the pain continues or persists, or if there are signs like
fever and increasing pain visit a
dermatologist.
Hope I have answered your query. Let me know if I can assist you further.
Regards,
Dr. Manoj Pradeep