Recurring swelling in left Armpit.

Hi doctor, I have been facing recurring swelling in my left Armpit since 8 months. It starts with pea size and itching then increase to marble size withing 1 hour if itch that swell or else...it remains same . After 4-5 hours, it goes off automatically .. without any medicine or treatment. It has happened 50-60 times in last 8 month's. Every times size varies. I have checked ultrasound of that area twice and results are normal. I met dermatologist and she suspect solitary mastocytoma and get skin biopsy done. I am afraid of biopsy fearing some cancerous ...does mine symptoms shows something cancerous ?? If twice ultrasound is clear...do I still need biopsy?? Please suggest. I met dermatologist when this swelling was not there in Armpit.