Is it normal for a snug piercing to make a bubble at the back of the ear?

It doesn't happen with every piercing but it's pretty common with cartilage piercings. Unless you have a history of keloids, its probably hypertrophic scarring. I cant be sure since I cant see it, so its a good idea to have your piercer look at it. Make sure that your jewlery isn't too short/tight. Also, be really careful with the piercing and avoid hitting it or playing with it.

To get rid of the scarring, try applying tea tree oil to the bubble. You can find it at a drugstore or target/Walmart. Dip a qtip in it, then apply it to the bubble. Don't actually put it in the piercing. do that 1-2 times a day until the bump goes away. The oil dries it up, so it'll flake off in layers. Continue to do your sea salt soaks as well.

Tea tree oil is really drying so if it makes the area really itchy and dry, cut back on applications. You can also use vitamin E oil to moisturize the area if it gets dry and uncomfortable. Hope this helps :)