Not necessarily cancer, there are many lumps in breasts that are benign in nature,but over the internet,it's very hard to even speculate what this lump could be.
You really need to see your physician for a clinical breast exam and some sort of tests to find out if this lump is benign or not.
I hope that it's nothing serious it could be a simple Cyst or Fibroadeoma but please contact your doctor for an appointment..a lump in the breast should never be ignored.
Best wishes and take care...