This could be a number of different things, but since you've had it for at least a few months, if not a lot longer, it's time to see your doctor.
I can't stress this enough. A lump on your neck can indicate a few different types of cancer. Obviously, I don't know if you have cancer, but you really should see your doctor to find out.
Hi, "Username". Just so you don't feel too worried, I'd like to point out the following to you:
Of all the people who go to see a doctor because of an unexplained bump on their neck, only a very small number end up having cancer. Maybe around 3%.
If it hasn't been getting larger, that's in your favor. Since you say that it is immovable, I'm guessing that you've done some research. Be aware that a lymph node being immovable doesn't necessarily mean cancer. We don't even know at the moment if it is a lymph node.
So, you can look at a visit to a doctor as the chance to rule out cancer.
Btw, is it round or oval? What's your age?
Oh, and have you been around any cats?