Thank you for asking your query at DoctorSpring.
It seems you are having a possible Colitis or infection in the lower intestine. The maroon stools might be normal, but a bleeding from the intestine should also be ruled out. Any swellings / tumors in the intestine is another concern. But there is no reason to worry as some basic investigation can throw light into the diagnosis.
The weakness / fainting could be either due to dehydration or anemia (low Blood count). If there is a bleeding from the intestines (which might be long standing and unnoticed) there could be anemia. Since you are 50+ of age, I would definitely recommend at least the following basic set of investigations.
1. A complete blood count
2. Stool Occult Blood
Your Doctor might prescribe you a course of antibiotics. Drink plenty of water. Make sure your blood glucose levels are neither too low or high (if you do home monitoring.). Hope this helps
Please Do feel free to ask followup questions.