Ive Had A Lump For A Little Over A Month

Ive had a lump for a little over a month now just above my left clavicle at the base of my neck. It went from pea to grape sized in that time and is still growing. It is moveable, slightly but hard, and it hurts but not as bad as you would think when u see it. Could endometriosis cause this? Also, I took a script of amoxicillin the first week, does this rule out infection?