non-painful armpit lump

I just realized what I had thought was "fat" in the hollow of my armpit is a lump. It is soft to firm kind of moveable but non- painful. I notice nothing in my breast. It is maybe about the side of a baby's fist. It seems to go inward into the hollow with arm in normal position and can only be seen when arms are raised over my head. It fills out the hollow of my arm on that side. No pain at all. I appear to be healthy. I had what the doctor described as a lipoma (fatty tumor benign) on back of the right shoulder behind the ball of shoulder. This lump is on the left side. I had a mammo normal two years ago and will be getting one the last of January. I am sooo afraid the diagnosis has to be cancer I am almost afraid to go. Is there any other possibilities other than some kind of cancer?