I am such a HUGE advocate of monthly self exams. That is how I found this! It was kinda funny, because when I found it the Dr's assured me it was a cyst. Then I had an ultrasound done and they said, we are shocked but it is a tumor. Then I had a biopsy.
It stayed the exact same for 4 years. But this year it changed - and I monitored it myself and alerted the Dr's. They said it was really important that I did because a fast growing tumor can be very problematic.
So yes, monthly self exams at the same time of the month and mammograms if needed are so important