Had sebaceous cyst removed from my neck a week ago by doctor, now have a lump there twice the size of the original cyst, how is this possible?

From a hematoma: It is possible that some bleeding in the area has occurred causing a hematoma. Also, it could be from an accumulation of serous fluid resulting in a seroma. Best thing to do is to go see the doctor who preformed the surgery.

Hematoma? Fluid?: The space that used to hold the cyst probably filled with fluid or blood. Should go down with time. If not, see your surgeon.