I wouldn't worry too much about it, i would worry more however if more spots develop.
Causes besides diabetes and
hypertension can be infections, a viral infection even , trauma
to the head, eye or chest, , anemia is a common cause , arthritis, lupus, and other systemic autoimmune disorders, pancreatitis, hmmmm - inflammatory conditions of the blood vessels etc. Even if you knew the infection you had as a child , it would be impossible to say this was the cause for sure.