(I know Luna is a kitty, but all my experiences are dog related, sorry)
My dog, RB Angie had a small lump in the same spot. She didn't die of anything related to it, so don't worry about that. Angie's turned out to be from trauma. When she would go outside she was so eager that she run while the door was opening and hit it, lightly, in the same spot every time. Maybe Luna is doing something similar? The vet drained some fluid and checked for cancer. It was nothing. I hope that Luna's turns out to be nothing. It could be a cyst or fatty tumor. Prayers are on the way for your girl. Because it moves doesn't mean you are in the clear with cancer, but it does make the odds with you that it isn't cancer or that if it is, it hasn't spread.
My boyfriend's 9 year old Golden just had a large cancerous tumor removed and went through chemo over several months. She is doing wonderfully and all her blood tests are clear. She also had cancer in the same spot when she was 2 years old. Just because they use the big C word doesn't mean the end. Try not to worry.