Lump on back of my neck?

There is a lump that is on the back of my neck on the left side right by the hairline, it doesnt hurt and i often forget it is there until i rub the back of my neck and feel it, it is about the length of a dime but a little thinner, when i push on it it does not hurt, it hasnt gotten any bigger ive known about it for about a month, does anyone know what it is?