What Causes A Lump On Underarms?

Hello I have a what appears to be a boil on my underarm, i got some stuff from the pharmacy about 3 nights ago, its called Magnoplasm its Magnesium sulfate-glycerol paste, i havent seen any changing of size as in i havent seen it going down at all if anything its gotten bigger... my mum also suggest that it could be a ingrown hair but someone also said that it could be a cyst... I havent tried to pop it or anything due to the fact that i am a sook and i am too scared to, i have also been making sure it cant get inffected! i have never never had something like this... could you assist me on this?