Thank you for asking Healthcare majic. My name is Dr Ehsan Ullah & I have gone through your query. Most of the lumps in armpits are harmless and are usually the result of abnormal tissue growth. However, a Armpit lumps can be related to a more serious underlying health problem. so any unusual lumps should be evaluated by a doctor. (Specially in female as mostly they indicate some breast pathology) There are various reasons for armpit lumps like bacterial infection fat tissue growths (lipomas) which are harmless noncancerous fibrous tissue growth (fibroadenoma) and even some Cancers like breast / bones (Leukemia) which have their different presentation and history. you are using hydrogen peroxide , although which settled this condition previously but it s not the case always. you just need a look on this lump by the physician, it seems an infective lesion so the Physician will prescribe you antibiotics along with pain & inflammation reducing drugs. after doing some work up like blood or pus tests. in Case of any suspicious look of the lesion the physician will do X rays and Biopsy of the lesion then treat this condition accordingly. sometimes , there is an enlarged lymph node in this region so that needs evaluation by the physician to role out its cause for proper treatment. Hope this may help you. Let me know if anything not clear. Thanks.