Hi, there is no way to be certain what the bump is without a biopsy. But it is very unlikely to be a lymph node, because lymph nodes do not naturally exist on top of a bone. The node would be very likely to get bumped and injured, repeatedly all through life. That said, an ultrasound can tell a LOT about what it probably is.
Did it appear almost overnight or grow slowly for years? A cancer can grow very slowly, but most likely wouldn't grow big quickly and then just stop growing.
"a collar bone that is bigger on one side"
I'm not sure what you mean there. It is overall thicker than the other one?