Why can't I wear earrings without my ears getting infected?

When I first got my ears pierced more than 10 years ago, I could wear any cheap earrings and it did not affect my ears. About a year ago, I noticed my ears getting red and puffy within minutes after putting in my earrings. They even bleed when I take them out if they have been in for more than a few hours. I have tried sensitive nickel free earrings, sterling silver earrings, and even 10 carat gold earrings and my ears still get infected and sometimes secrete pus for days after the earrings have been removed! What is wrong and why can't I wear my earrings?