This looks like an access typical of hydadenitis suprativa. You should have it drained. If it doesn't go away you might need to have the infected gland removed. MiraDry should help prevent this problem.
I had miraDry 2/5/16 but I keep getting a cyst under my left arm. Are hot compresses or cold better? (Photo)
Doctor Answers 3
Cyst under arm
Hi there. This is a cyst and it should be drained so it gets better sooner. You should see your treating physician as soon as you can.
Best wishes, Dr. ALDO
Retained cyst after miraDry
Hopefully, a surgeon did your miraDry and he/she can drain it.
Dr H Karamanoukian