Cancerous lumps don't hurt - they have no nerve endings. I learned the hard way by finding a swollen lymph node on my collarbone a few weeks after my daughter's birth and was diagnosed with Hodgkins lymphoma a few weeks later.
If you are even remotely concerned, don't ignore that little voice in your head. When we were trying to figure out what was going on with me, my OB/GYN and Family Practitioner both referred me for breast ultrasounds - all of which were clear.
Peace of mind is priceless - if you make a call, they will direct you appropriately based upon your medical history.
Good luck. I hope it's scar tissue and nothing of greater concern.