So painful. Is this a pimple?

See pic below... it's in my underarm. Pretty damn painful, feels like a boil. It's been here for like 3 days and doesn't have like a pimples head to it and when I squeeze it nothing comes out. I just tried tea tree oil tonight. Last time I had this was I my inner thigh and it felt the same way but when I squeezed, a lot of pus came out so then I put tea tree oil on it and the next day it was completely gone. I'm just worried about this here... should I be worried? What is this? 😳