Unspayed dogs or dogs spayed later in life are more suseptable to mammary tumors and this might be what you are feeling. You need to watch for a change in size,shape, texture and color. Any changes in those then you should have it removed. It could also be an abscess or just a fatty growth but like I said, watch for those signs or if it starts to bother your dog then you should see your vet.
Good luck!
Let me know if you have any other questions
Good Luck &
Best Fishes