Thanks for the query.
Sometimes medicines may alter the menstrual cycle.
But your irregularity in periods can be more possibly due to the health issue you had and for which you have taken metronidazole
Recent health issues can alter the hormonal
levels and can affect the menstrual cycle.
If you had unprotected intercourse
during the period of ovulation, go for pregnancy
test once to rule out that possibility.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429