I was washing my palomino's tail today, and I came across a lump at the end of her tail bone.  It was covered with white crust.  After gently peeling the crust off, the lump it about the size of a silver dollar, oblong in shape, and lumpy.  She has also lost the hair in that particular spot, but it doesn't seem to bother her any.

Has anybody experienced this before, and could it be cancerous?  I have a vet appointment next week but was looking to see if anybody could fill me in until then.  Thank You.