Why the weak erections?

Hey all. Last Friday, I was overstimulating myself and keeping myself hard for a long period of time until I felt a short burst of pain in my penis. I decided that it was time to stop, so I did, but I made sure I masturbated to get the semen out. Well, it's been almost 5 days since that time, and I can't seem to maintain a solid, firm erection like I used to. I'm really scared and I don't want to lose my potency at the age of 20.

Help?