Fresh rectal bleeding is relatively common, and if a single episode, particularly in the context of constipation or a sensation of stinging/burning around the anus during passage of stool, unlikely to be of concern.
However, if dark red blood, mixed in with stool, recurrent episodes, or associated with other symptoms, e.g. change in bowel habit, weight loss, it is important to seek medical advice.
It is likely that you will be recommended for a colonoscopy or flexible sigmoidoscopy.