If you masturbated dry (did not use a lubricant) and the bump is near where you were rubbing your penis, then it is most likely a common blister. Give it time and it will heal.

If you are circumcised, you should always use a lubricant when you masturbate. If you are intact, it is common to slide the foreskin over the penis to masturbate.

Maybe a blister, which should heal. Perhaps a cyst?