Your stoma is made from the same type of skin as the inside of your cheek and you know how easily that bleeds. As long as the bleeding is from the outside of the stoma it should stop relatively quickly. You may have just nicked it as you were taking your wafer off - it's nothing to worry about. Even brushing it slightly with a towel can cause it to bleed sometimes since the stoma is full of little capillaries.
It's when the bleeding won't stop or is coming from within your stoma, that it's time to seek medical help.
I have had an ileostomy for 30 years now due to UC.
I'm not a complete idiot - some parts of me are missing!