Any number of things .Could be something you have eaten which gives the appearance of blood .Could be that you have a pile .Could be because of any medication you are on which can cause ulceration which leads to bleeding perhaps, or a disorder in the digestive system .
But ...if you are worried and it is ongoing please see your GP as soon as possible to determine the cause .
you dont say if you have stomach pains etc.
three weeks ago i visited my gp sufferihg bad pains and black tarry poo.
i have arthritis and was on medication that seemed to coincide with my symtoms.
my gp reffered me immediately to hospital and tests reveiled the medication was the cause and i was put right.
when in hospital i soon became aware that any form of internal bleeding is taken extremely seriously.
you must see a gp asap and i do hope you soon get sorted.
all the best