Concerned about bump deep in hind quarter

I was giving Heinz a massage and noticed a bump about the size of a marble deep in his hind quarter muscle.  I just noticed this two weeks ago.  We have a dental appointment coming up and the Vet said she would check it for me, but I am concerned.  He is 12 years old. It is hard and doesn't seem to move.  Has anyone else had anything like this with there cats?

Thanks!

That's not something I have ever seen before but maybe someone else has. Hopefully it's nothing serious!

 Vibes that Heinz checks out okay. Please let us know what the vet says! 

Rosie just had a benign hard lump removed from her shoulder,  but that was because her microchip had moved and caused a tissue reaction.

I hope  your babies is something simple 

Could be a sarcoma(?). A reaction to a vaccination usually the less expensive kinds some vets use. Our cat is going in Friday to get one removed fom his hip. It feels almond shaped and not attached to muscle. The vet says its a simple removal. Our boy will be home same day. But it is important to get these removed because they are cancers and it is important to get rid of them before they spread.

Thank you!  I am taking Heinz (the smaller one in the picture...although he is 22lbs) to the Vet the 1st of March.  He is the most precious, sweet, loving cat I have ever owned.  Hanz his brother is close to 30lbs.  They are very long and big cats.  Their mother was a a very small cat, she never really got much bigger than my hand.  Hanz is aloof but such a huggy bear at night...that is our time and very sensitive.  I love him so much as well. 

It feels like it is solid and hard.  He doesn't like it when I try and find it.  I have an appointment with the vet.  I am just praying it isn't thing serious.

Thank you for your concern.  He is my baby...he acts like a baby.  LOL   His brother is more the King of the house and this one is the one that hunts - brings me toys and wants my lap all the time.  I sure love my boys!

Keep us posted.