Women can be late for a variety of reasons and illness is one of them. Needing antibiotics means you've been sick recently, so yes, that can be the cause of why you are late. Since you are on the pill AND used protection, pregnancy would not be the case. :>) Other reasons women are late are changes in routine, weight gain or loss, stress or just a blip on the hormonal radar. Every woman is late at one time or another. good luck