cancer tumor or just being pariond ?

I am 18 years old and for the past few months I have had this lump in my right near the side by my arm pit. It's only painfull when I push harshly on it .this doesnt happen on the other so i ume this is bad. I told my mom but she doesn't seem to care enough to make mean app. I have no money to go to a doctor but i m afraid that this could be cancer .. I'd rather play it safe and live than keep ignoring this .. I heard caffine plays it's role in some of this i do drink more soda than the usual person will drink in a day if that helps at all