I Have A 9 Year Old Black Lab. She Is

I have a 9 year old Black Lab. She is very healthy. Eats well. Drinks plenty of water.normal body waste functions. Sleeps soundly through the night. Is a fun...warm...family pet.
A month ago I had a growth removed from the hind leg. It started out as a small growth...which I initially thought was a tick..but could not see legs or movement under close observation. I kept an eye on it and over a span of 6weeks it grew to the size of a small thimble and she was licking it sometimes. One night before going to bed I noticed it was red at the tip of it....it was a small drop of blood.
I took her to my vet 3 days later and she removed it.
She said it didn't appear cancerous but simply a cell mass.
She healed well and hasn't missed a beat...or a meal..through the whole ordeal.
Now I've noticed a small mass directly under the skin where the original mass was removed.
It is very soft....movable slightly..fells like an under inflated water ballon about half the size of an egg.
She never bothers it...causes not pain or discomfort when I examine it and move it around.
I'm afraid it might be cancerous and if so I don't want to put her through any pain of unknown treatment.
Like all older pets....she is an extension of my family.
She is 63 ....in human years....I am the same age.
She is my best friend and knows me as I know her.
What is your best thought on her issue?