What Causes Lumps In The Blood During Menstruation?

iam 24 year old i had an abortion of7th weekin aug then on ward every month 5 days befor periodsits bleeding a little bit and in nov itbleeded or 15days in dec i missedmy periods in jan it bleeded normaly for 5 days but after thatonce for every 1 or 2 days it bleeded in formof lumps upto15th day in feb 24th i got my periods and its bleeding normally until now and bloodis containing lumps plz tell me the problem