Based on your description, I feel confident in saying it is more than likely an ingrown hair follicle. This is esp true if they are of rapid onset. This is nothing more than an infected follicle that will heal by itself or if too bothersome, you can pinch the area to force it to come to a head same as a facial pimple and use tweezers (dip in alcohol first) to withdraw the hair which has more than likely encircled itself in the lump.
As far as prevention, get some facial scrub like Neutrogena scrub and vigorously scrub under arms and bikini line area twice a week. If you notice any ingrowns hairs, you can attempt to squeeze and pluck them out and they will then heal spontaneoulsly. If you happen to be waxing, this practice actually promotes the problem, but can be treated as above. Also, try not to wear extremely constrictive clothing, as this may make matters worse. Allow your skin to "breath". In addition, be sure to use an alcohol based astringent and swab affected areas daily.
In the event that it enlarges and does not appear to be infectious in origin, you may consider visiting your PCP or dermatologist. If you are able to move the lump and it does not seem to be securely rooted in place, it should subside on its own. In the event that you are continuously afflicted with these complaints, may want to ask your dr about a low dose antibiotic such as tetracycline, esp if facial area is also involved. Good luck and LMK if you require addtional info
I have an advanced nursing degree and years of ICU experience. What I don't know, I can research further. Of course, nothing replaces an actual physical exam by your physician. Be well, Stephanie0107@hotmail.com