Yesterday I noticed a very hard lump on my collar bone. Its so hard it almost feels like a bone. Could this be cancer?

?subluxation. Look in the mirror. Is there a prominence in one collar bone compared to the other? If so sometimes the collar bone can move as it is a joint. Usually occurs at the sternum at the sternoclavicular joint. Usually not painful. Nevertheless, have your doctor look at it to be sure. Hope this helps.