You may possibly be looking at what is referred to as a fracture bump from a previous (unrecalled) injury of the clavicle (collar bone). The only way to know for sure is to have the area X-rayed. But if you are not experiencing any pain or other symptoms, X-rays may not be needed. Unnecessary exposure to radiation is always to be avoided.
The age at which fractures of the main portion of the clavicle (called the clavicular shaft) occur is a key factor in how it heals and looks. Bone remodeling is more likely aat younger ages. For example, after age 10 (for girls) and age 12 (for boys) remodeling is less likely and less predictable.
There is also a possibility that you have an uneven pull of the muscles that attach to the clavicle contributing to this bump. There are any number of reasons why this type of asymmetry might occur (e.g., posture, malalignment, difference in clavicular length from one side to the other present at birth).
Consider seeing your physician if you are experiencing shoulder pain, loss of shoulder motion, or loss of shoulder strength. Otherwise, chalk this one up to unknown forces/reasons.