How come I get allergic from wearing earrings that I have worn before and caused no reaction?

Possibilities. .. are threefold. 1) not an allergic reaction to the ear ring; 2) you have since developed a sensitivity since the last time or 3) the plating on the jewelry has eroded and contact with an underlying metal is now taking place. Testing for metal and other contact allergens is available and can guide management strategies. An Allergist can help!