Hi..
Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concerns..
As per your complain it seems that that as you had hard stools, there is a possibility that during straining to pass the stools there has been trauma to the mucosa of the anal canal while passing the stools leading to slight bleeding..
It can occur commonly during 
constipation..
As of now you need not to worry as it happened only once..
In case if you still experience hard stools you can take a high fibre diet and can also take 
fibre supplements like Psyllium as it helps in  increasing the volume of stools as well as ease in withdrawal..
You can also take laxatives like 
Polyethylene glycol and 
stool softener like Docusate..
Drink plenty of water.. 
But in case if there are repeated episodes of same problem that is bleeding and if you start getting discomfort in anal region you should 
consult a Proctologist and get evaluated and he can do a clinical examination as well as do a Colonoscopy or 
Sigmoidoscopy and confirm the cause as certain conditions like 
Anal fissure, Hemorrhoid while some disease related to digestive system like H.Pylori infection, ulcers etc can cause bleeding in stool..
Hope this information helps..
Thanks and regards.
Dr.Honey Nandwani Arora.