How can you not know?

Went to the gym yesterday; spent 45 mins. on the crosstrainer and some weights. I should have gone for a run really but I'm coming down with a cold. Went to see Dr Poon this morning to get something for my sore throat and told him about my low haemoglobin level and tiredness. He sent me off for some tests. One is to test for blood in my stools.

Why?
Well, maybe you've been shitting blood without knowing.
How can one be shitting blood and not know?
Well, if there is a minor leision up your arse then blood would be coming out with your stools slowly and you wouldn't notice it cos it won't hurt and the blood won't be gushing out.
....er... okay.


I'm not sure I caught his drift completely but I'd just have to take his word for it.