I’ve just found a lump probably the same size as a marble under my left armpit, it caused no pain today during college but i’ve just got gone and changed and felt an uncomfortable pressure under my arm and felt a lump. as a 17 year old female this is worrying to me as i don’t feel any different anywhere else on my body? 

im hoping it will just be due to irritation from shaving/deodorant etc but just want some advice right now

thanks :)