Hello,
From your symptoms it appears that you must have masturbated harshly.
Sometimes when you tend to masturbate harshly, you could have damaged your frenulum which has led to bleeding. There is nothing to be worried about as it must have healed by itself by now. But, sometimes there is history of excessive bleeding from frenulum tear. As an artery passes in sub mucosa of the frenulum. Due to excessive pressure there are chances of bleeding.
There are chances it can occur again, I request you if there is excessive bleeding to visit a doctor at the earliest, you need to apply artery forceps to stop bleeding.
Thank you.