It almost certainly will be nothing SB. I might be wrong, but if its a cyst or similar, I would expect thickening anyway - you know when you have your babies vaccinated and they get that little hard lump at the injection site? Well I would think a cyst would generate the same response, hence the thickening. Any proper medics on site able to confirm??? Besides, we all get stuff like cysts, boils, cold sores etc when under stress - and no one can say you haven't been under a smidgeon of stress lately can they?
I know it sounds dreadful SB, but rather than these vague nameless worries, focus on the worst case scenario ie its a cancerous lump, then think thru what you will do if thats the case (which is won't be). Well for starters you are young and healthy, secondly you have obviously caught it early, thirdly you are not thick and can arm yourself with all the available information and fourthly, treatment for breast cancer is much more advanced now and is eminently curable. My mum had it 37 years ago - she weighed 5 stone when they operated and she's still here.
But, it is almost certainly nothing to be concerned about, okay? But leave it alone if you can
Email me if you want, I shall be sat here prodding at my lumpy boobs too