Well, an erection is when a guy's penis is hard, and that's necessary for intercourse, so I think what you're referring to here is ejaculation, when semen (containing sperm, of course) is released from the penis.
Yes, m'dear, you do have a pregnancy and STD risk here. If it's been less than 100 hours or so since the incident, please get to an emergency room, sexual health clinic, or walk-in clinic for emergency contraception. If it's been longer, you need to find a sexual health clinic (a lot of municipal health units have them, check your phone book), or make a doctor's appointment to be screened for pregnancy and STDs.
And, if alcohol is enough to make you do risky and unplanned things, which would seem to be the case, you need to stop drinking. Especially if you're not old enough to legally drink. There's nothing to gain from something like this, and a hell of a lot to lose.
Milke, with an L, SSBD, RATS, TMNTP, MF
I still love you, oh, I still love you
...Only slightly, only slightly less than I used to