What Causes Bleeding After Embryo Transfer?

hai, ihad embrayo transfer 11 days ago,very very scanty blood discharge i had before the transfer, chenged to spotting bright red blood on 7th day of transfer.today is 11th day having perid like bleeding started.i am 38 year old.i had bilateral salpingectomy.i have one daughter. i got hypothyroid.i had ivf tretment.two good quality embrayos transferred