I have very lumpy, cystic breasts that sound very similar to what you are describing. Generally, fairly large lumps that are moveable are cysts and are not a danger. Usually, cancerous lumps are not moveable and actually tend to be smaller.

Those of us with cystic breasts almost always need ultrasound along with mammograms. I am always given both. Both of my sisters had breast cancer though, so they don't take my exams lightly and insist on two a year.

The time I did need a biopsy was for a spot that was not at all palpable. I had gone in for a large, painful, moveable lump on one side. They were not at all concerned about it (and it was just a really large cyst although they went ahead and aspirated it because it was painful and had hung around for over 3 months), but in the process of investigating, they found this very small spot that was too deep to have ever been felt. I had the biopsy and it did show to be precancerous, but it was removed and marked and is not a concern now.

I've learned to not get too worried about things until they actually say it's time for a biopsy.