Kinda sounds like a follicular cyst. These start small, and can keep growing. They are usually hard, and movable. They are benign and can be left alone...for a while...or removed right away. When small enough, they can easily be removed with a biopsy punch, and then a tiny stitch is put in. Even though benign, it's a good idea to send it in for analysis.
My Vet Derm says "if it's worth taking off, it's worth sending in"
.