I would suggest that you bring this finding to the attention of your primary care doctor
, who will be able to help you figure out what is going on and decide whether or not any additional treatment or work up is needed.
I can think of a few potential causes, which your doctor can confirm. One is a lipoma, which is a benign tumor composed of fatty tissue that grows beneath the skin. Lipomas have a soft rubbery texture and they move freely under the skin when you touch them. They are benign, and do not generally require treatment unless they grow or become painful, at which point they can be surgically removed.
Another strong possibility, based on the location of the lump, is that this is some scar tissue. This is very common on the shins, because they are frequently bumped or bruised. Even if you cannot recall a particular injury
, it is possible that some unremembered bump was enough to cause scarring under the skin.
Statistically speaking, more serious problems, such as a cancerous tumor, are unlikely. However, your doctor will be able to examine the area and if there are any concerning features (for example if the lump is hard and fixed and doesn't move), they may recommend that you get a biopsy