Do I have a cyst in my breast or is it something more serious?

Im 19 years old and I found a lump in my left breast in the outer part near my armpit, not on my armpit! both breast are experienceing pain and swellin because I should be starting my period soon and swellin breast is a normal thing for me during my cycle, the lump doesnt bother me unless i press on it. breast cancer does not run in my family, Im not over weight. i dont have any health issues, but im like freaking out over this and i cant sleep at night. my mom told me that shes had cyst in her breast and im wondering if thats what i have, but is it supost to be painful? i dont have any dimpling, inverted nipple or and change in skin color, just swelling but once again im supost to start my period soon. iv done alot of research on the internet and found alot of stuff like dense tissue or a swollen lymph node, but i wanna know if there are doctors out there wh ocan reply and maybe explain this to me or someone who has had the same thing. please help!! :\