Hi and welcome to the Chiari forum.
If I am correct your surgery was for a disk issue and not Chiari?
The reason I make a point of that is bcuz it may change why you have a lump....and what you might be able to do.
First it could be a muscle that has twisted....and created a lump....so some PT may be helpful...but see your Dr first.
Second it could be a leak if the dura was nicked while you had surgery....
Third it may be scar tissue forming....some of us develop it faster then others.
Have you had any PT since your surgery?