Dear misskaren: This lump could be many things. It is unlikely, in the location you describe, to be breast cancer. If the lump has been there 4 months, it needs to be evaluated by a physician. This is especially important if it is getting larger.
There's really no way to diagnose a lump without examining it. Any newly appearing lump ought to have an answer; lumps that are visible or feelable in or just under the skin, which are not within the breast or in the neck itself are most likely to be pretty innocent. Possibilities include a sebaceous cyst which is a plugged oil gland, a lipoma which is a harmless fatty tumor; it's rare but not impossible to be something more serious, and since it wasn't there 4 months ago, it makes most sense to have it looked at.