Lump at base of tail

My male cat Monty (age is about 11) has a golf ball sized lump/mass at the base of his tail on the side. It doesn't seem to cause him any pain or discomfort when I touch it. He doesn't hold his tail down and seems perfectly healthy other than vomiting occasionally. Any ideas as to what this might be? Thanks!

Just to be on the safe side, I would take him in to be checked.  No discomfort sounds good, but the size of it doesn't sound too good.  I'm guessing it must be growing

BTW, I have absolutely NO idea what it might be....don't think I've heard of a fatty tumor getting that big.  Here's something about Fatty Tumors, just in case:  http://www.petmd.com/cat/conditions/skin/c_ct_lipoma#

Thanks for your prompt response. Upon further viewing his mass is more the size of a large gum ball (rather than a golf ball) and seems to be perfectly round.
A friend first noticed it about a week ago. I have no idea how long it's been there. It went unnoticed by me because my hand doesn't generally come that close to his behind, lol.
I'm going to be our of town until mid-December but will get him to the vet after that.