What could cause a large liver like tissue in my period?

Usually from large : Blood clots caused by excessive endometrial tissue build-up assuming you are not preg, you need to see your gyn and your gyn can do blood work to r/o hormonal causes of this and an ultrasound and you may need an emdometrial biopsy , if endometrial tissue is n, and if u do not want fertilty , I do emdometrial ablations in my offcie - 8 minute easy proced to get rid of the tissue so no more bleeding.