What is the appearance of the lump?
What is the size?
Is it warm to touch or tender?
A small lump under the skin may be a fatty deposit called a lipoma however if it is large then we would want to rule out a hernia, although the location is not typical for hernias.
How long has it been present?
If the area is slightly swollen then I might consider infection or abscess.
If the swelling goes away when you lie down then I would want a check to exclude hernia, just to be on the safe side.
As you have had it about a week with no improvement then it needs to be examined to determine what this might be so you will have to see your doctor for examination.