I have pain in the lower left side of my back , dark stools and constipation?

Needs evaluation: Especially in your age group, there are many concerning conditions that could cause those symptoms. You need to speak with your physician, and if you have not had one, get a colonoscopy.

Time to see doc : If established with a primary care doc, start there. If already established with a gastroenterologist for a specific condition, starting with them would be even better. Bleeding and a tumor will have to be ruled out and constipation addressed among others.