Why is my earring making my ear sore?

I'm wearing a surgical steel earring in one ear, and it's a little sore if I press it or pull on it, but it's not swollen or red. the other ear has a plastic earring in it, and it feels the same but not as sore. they both have larger posts too, 16g , so it's not that much larger than the average. I've had my ears pierced for 15 years. I usually only wear gold or silver earrings, because I don't know if I'm alergic to certain metals, but I decided to experiment(that's why I'm wearing two different earrings, lol) thanks in advance.