Everone's testicles retract right before orgasm. If you masturbate with your legs apart, and you don't have low hangers, they can retract right up into the inguinal area. No big deal; they drop back down, or you can push down on the pubic area and get them to pop back out.
I would say you're perfectly normal. It's quite common for your testicle to retract slightly when you orgasm/ejaculate. Some men do not hang as low as others so its more noticeable when they do retract. I'm guessing that your left testicle is just more susceptible to retracting than your right.
But for an expert opinion, mention it to your doctor at your next physical.
i forgot to say but i am 18.