How can i know if this is a allergic reaction or infection from my pierced earrings?

Not always easy: Allergic reactions to earrings can present with itching, swelling of earlobes, tiny blisters and oozing. T^he best thing to do would be to remove the earrings for a few days. In most cases it will resolve without treatment or with local compresses. No better, see a physician. If the reaction recurs each time you wear your earrings, it is probably allergy, usually to nickel in costume jewelry.