How can tooth with root canal hurt? It was done 8 months ago (in 2 sessions, regular and with microscope) crown replaced old one with decay under it.

Root canal failure? A root canal treatment can fail. Even the best root canal specialist doing the root canal to the best of their abilities can still fail. I would have the area reevaluated as there could also be other issues.
Maybe something else. Is it tooth or gum? Maybe pain is coming from somewhere else. Do you have fever/chills/bad breath? I recommend seeing your dentist to discuss this...Or call the dentist who did the root canal to discuss.
Recall visit. Recall visits are standard and part of treatment for this very reason. At that visit, your dr. Will answer your questions, take an x- ray, examine the surrounding tissues, and check the bite. You will be told what is going on and what may or may not be needed to solve it.
Phantom Pain? It could be what is known as phantom pain. This is often felt when a leg is removed yet the patient may "feel" their toes. It can happen with teeth that are extracted or hace rc treatment. Also, the crown could be high or hitting incorrectly causing pain to the periodontal ligaments. See your dentist to eval the bite and check for signs of other problems. Tooth may also need apioectomy.
Faling root canal. Root canals are not always succesful, they are actually pretty tricky sometimes. Even though the tooth no longer has a nerve, the surrouding environment has plenty of feeling and the pain might be coming from the ligament that holds the tooth in the bone.
Numerous reasons. The best rc specialist in the world may achieve a 96-97% success rate, so 3-4% of root canal treated teeth are going to fail during the next 15-years. Treatment is a last ditch and often imperfect effort to save a doomed tooth from extraction. Best bets: change the habits that caused the need and be pre-emptive in upgrading your dental work. Fillings are temporary fixes for tooth decay/fracture.