I'm a regular 16 year old with a regular uncircumcised penis. Yesterday I did an activity that made the foreskin push back underneath the head. So now my foreskin is underneath the head, but it was pretty painful and is now painful when I get an erection. What is happening, why, and what can I do to make sure it doesn't hurt when I get an erection?
Do you mean that you left your penis unsheathed so the glans (the head of the penis) remains exposed? If so, you need to return your foreskin to its regular position. Unsheathing your penis is supposed to be only for short periods of time. If it is gets stuck under the glans, it can lead to a condition called paraphimosis. The foreskin, because of the lack of circulation, swells making it even harder to return to a normal position. The swelling foreskin is painful and can cause problems. See: Paraphimosis: What It Is and How to Prevent It.
The trick to getting it back into position is to press on the glans, not the foreskin, so the foreskin can be rolled forward. If you can't get it back yourself, you will need to see a doctor.
If you have already gotten your foreskin back into position and it still hurts, my suspicion is that the opening of your foreskin is tight and you ended up stretching it. Even after it goes back, it will stay sore for a while. Instead of trying to stretch it all at once, you should stretch it a little bit at a time. See: I can pull my foreskin right back, but when it is pulled back it is really tight.