Is it normal that one one of my balls retracts itself?

Coming in for a landing.

I guess it's normal. My boyfriend's does it. I like to tell him something along the lines of "ooh! I emptied out all your cum!"
XD

Completely normal. Some guys have both balls go up. And a little short-term pain is also normal. Careful if this happens when having sex because your nut in that canal could get in a certain spot and get smashed between your pelvic bone and her. OUCH! As long as it goes back down, you're good. If it stays up for days, time to see a doc.

I've seen that happen in a couple of amateur porn videos, so you aren't unique - unless you're the guy in the vids.

More seriously:

https://www.netdoctor.co.uk/ask-the-expert/sex-faqs/a11844/any-help-for-retractile-testicles/

JD777 is right. Don't worry, but why didn't you ask around a bit? If your eye went a bit droopy or you got a rash on your leg you would ask. Before you dismiss this question out of hand have a good think.

That happens to me bro, good to know I'm not alone. About to go hard now and see if it happens again.

If you stop jerking off so much this kinda stuff wouldn't happen.