I have had what looks like my lining of my uterus come out but i haven't bled just spotted two days before but only for that day. why could that be?

Maybe polyp? period?: First of all, be sure you were not pregnant. but if that is neg, then you likely shed a polyp. sometimes the whole uterine lining sheds ( called a cast..) but unclear why that would happen just at that time. unclear if you were due for your period. Bottom line is if recurrent abnl bleeding happens ( like more than 7 days or more than 3 heavy), then see your gyno to evaluate.