Hi.
The lump in your armpit which has been there for over a month could be many things..like a swollen lymph node,a hair follicle infection, a cyst etc.. I think that the proper thing to do is to have this lump checked out by a doctor who can examine your breasts and maybe order an ultrasound or other tests to make sure what this lump represents.I could be nothing serious,but it's impossible for us to tell you over the internet what it is..You really need to see your doctor just to make sure that you are okay.
Best wishes to you...
Without benefit of examination it's impossible to say what this lump might be but from the location you describe it sounds like it could be a lymph node. First I would advise you to STOP pressing on it as this may well be a source of irritation. It could also be an infected hair follicle. If you are terribly concerned then see your Dr. and have it examined. If it's been present for the past few months I doubt that it would be anything too serious. Regards .....