Lump on dogs leg

Hello,

As i was petting my dog today i noticed a bump on his back leg. The only thing i can say is that it was the size of a large marble and grey. It did not bother him to much to touch it but i could tell he did not like me doing it. It did not cause him pain, but i'm really scared and i need to know what you think it is.

Thank you!

Lacy Holmes