What Causes Lump Under The Armpit?

i have a lump under my armpit that grew from Friday until today. I hve not had an ultrasound on it yet. it is sometimes painful but not too bad. More of an irritation. Doesmt itch. I am allergic to pennecillian. So the nurse practitioner prescribed me cephalexin 250 mg. Reading up on it it says not to take if ive had an allergic reaction to penicillin. I am allergic to penicillin. should I still take it and what is this lump?