Dear remor518: A lump that comes and goes is not generally cancer-related. If it is also red and sore, it sounds more like an ingrown hair or pimple. If a lump stays more than a few weeks, gets larger and doesn't go away, you should see your doctor for an examination.
something that comes and goes is almost certainly not cancer. It's not rare to have recurring infections in sweat glands under the arm, and since you see redness that makes it fairly likely. It might need surgical drainage to make it go away permanently. Also, using deodorants that don't have antiperspirants, which plug up the glands, can be useful. But to have it accurately diagnosed, it'll need to be looked at by your doctor.