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I recently had the same thing. After my research I have found that it may be an infected salvatory gland caused by a coinciding cold sore. This makes sense, since I didn't have it before the cold...
If this "large lump" is a cyst it may eventually burst. There is nothing I know of that you can do other than wait for things to take their course and make sure the fishes water is kept perfect by...
I regularly had these as a teenager and sought medical advice as a result. They're almost certainly blocked pores or spots and will probably go down within a few days if you keep your armpits...