My Psychiatrist Gave Me Sertraline Hcl 50 Mg And Told

My psychiatrist gave me Sertraline Hcl 50 mg and told me to take it a week before my menstrual cycle and then stop once my menstruation starts. I am already on 300 mg a day of bupropion er. How does as needed sertraline help with PMDS? He also told me I could take Zoloft as needed, but I am thinking it will do nothing as seratonin has to build up before it takes effect.