Should antibiotics cause a prolonged, light, and early period? I've read that they can cause late periods, but mine was just the opposite. It came a couple weeks early, it was very light, and it lasted through the time of my normal menstrual cycle. Can antibiotics do this? If it helps, I started a 10-day course of 20 amoxicillin tablets right after my last period had begun and also received an antibiotic shot about two weeks later. So yeah, do I need to be concerned or is this probably normal?