I have been taking doxycycline for severe acne a month now. i have had lighter spotty periods since jan. period is 2 days late this month, why?

Not doxycycline: This is very unlikely to cause menstrual irregularities. Especially since you have severe acne, there is likely to be an overall problem with your hormonal milieu and cycle. Your physician is the best person to discuss this with. Heads up -- no antibiotic should be the sole therapy for acne; i trust you are using a topical agent as well. Best wishes; glad you're staying on top of these issues.