Hi, Usually many antibiotics will not interfere with the reproductive hormonal balance. But the cause for which antibiotics where used can interfere like the sickness of the body by affecting the metabolism can lead to reproductive hormonal imbalance and menstrual irregularity. Also, while using oral contraceptives amount of menstrual irregularities can be seen like decrease blood flow during the periods called hypomenorrhea, inter-menstrual spotting, etc. As you have excluded the possibility of pregnancy, no need to worry, you can continue with your contraceptive pills and possibly your cycles may regularize soon. If the hypomenorrhea persists, better to get evaluated by undergoing investigations like hemoglobin levels, blood sugar levels, thyroid profile, serum prolactin levels and ultrasound. With this work up the possible cause of your problem can be identified and treatment can be taken accordingly. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Sree Gouri S. R., OBGYN