I have one that I'm in the process of getting tested (waiting on ultrasound results at the moment). Mine is small and firm, moves slightly from side to side when pushed on and although it can be more difficult for me to find based on the position I'm in, I can still feel it and the size hasn't changed.
For instance, when standing or lying somewhat on my left side, with my right arm above my head, it is easier for me to feel than it is if I'm lying flat on my back. I am guessing this might be because I'm large chested so gravity pulls everything downward when I'm lying flat
It is impossible to tell the difference between a cancerous lump and a benign lump just by feel alone, although there are sometimes certain "characteristics" of cancerous lumps. All the reading I've done says that *most* cancerous lumps are firm, irregularly shaped, fixed in place.
If you have a lump, no matter what you might think the cause, go to your doctor and get it checked out!