Nothing to be worried about.
Thanks for your query,based on the facts that you have posted it appears that you have noticed a prominent vein in probably on the dorsal aspect of foreskin.
In many individuals there are prominent veins which are tributaries of dorsal vein of penis .
These prominent veins do nor have risk or side effects on sexual potency and fertility
of the man.
Please do not worry .They are neither cancerous nor infective in nature and are not due to masturbation
Hope I have answered your query ,please feel free to ask if you have more questions ,
I shall be happy to help you
Thanks and Regards.