One possibility is that you have a lipoma. A lipoma is a soft, fleshy growth of fat cells under the skin. It is usually freely mobile under the skin and can continue to grow over time by not always. A lipoma is a benign condition and it does not need to be treated unless it becomes painful or is cosmetically disturbing for you.
There are a number of other growths under the skin in addition to lipomas which would also be possibilities. These include various types of cysts and growths. Fortunately, most of these are benign and do not need to be treated.
However, I recommend that you talk to your primary care doctor
. They will be able to diagnose whether this is a condition that requires further investigation or not.
If your doctor feels that the growth is most likely benign, they will probably recommend that no treatment be pursued unless it is bothersome to you. The only effective treatment is to surgically remove the growth, which is generally a simple procedure which can be done in the surgeon's office under local anesthesia only. If, on the other hand, your doctor feels that the growth should be investigated, they may recommend removal or a biopsy