Hi and thank you so much for this query. Trimethoprim does not increase the metabolism of drugs and thus would be very unlikely to be responsible for this observed changes. Because this is the first time you have experienced this, I will like you to observe and see whether it continues or not. Normally, persons on this pill will not have any menstrual flow. For how long would you take bactrim? Because it is always a short duration of treatment, you should continue the treatment to completion. Thanks and hope this helps. I wish you well. Feel free to ask for more information and clarification if need be. Thanks.