Why are my earrings making my ears itchy?

I have a pair of earrings that I wear consistently, almost every day, and they've never made my ears feel itchy or uncomfortable. Yesterday, I wore a different pair of earrings (I don't know if they were sterling silver or alloy), but they didn't make my ears itch. Now today, I go back to wearing my good ol' earrings that have never made my ears itchy previously, and now my ears are itchy and red! I took them out and my ears are still a little itchy/uncomfortable. What happened?