What Causes Prolonged Bleeding While On Femara For PCOS?

hai i m facing irregular periods ,i recently diagnosed by pcos,i undergo laproscopy surgery too.my doctor gave me a metformin,femara.after using metformin i got period,on the 3rd day i went to check up my doctor said everything is alright take femara medication now.but yesterday is the 5th day of my period,still i m facing bleeding problem. may i know the reason why i still getting bleeding,is this is a serious?