There are a couple of different possibilities here, and the best thing to do would be to have the area examined by your primary care doctor
at your convenience. They will be able to examine the lump and tell you what it is likely to be and whether or not it will require any treatment.
One common cause of the type of lump you describe is something called a lipoma. This is a form of benign fatty tumor which can occur anywhere under the skin of the body. Lipomas do not usually cause any major medical problems, although they can sometimes get large or uncomfortable and require removal.
Similarly, this could be a cyst, which is another type of growth under the skin, often a bit firmer in texture than a lipoma. Like lipomas, however, cysts are usually benign and do not often require treatment unless they are painful.
Your doctor can examine the lump and confirm whether it is a cyst or lipoma, as well as consider alternative diagnoses. If they decide that this is something that should be removed, this can usually be done as a simple office procedure with some local numbing medication.
If the lump is growing rapidly or changing in any way, you should get in to see your doctor as soon as possible.