First of all I think u haven't had a crown done after the root canal . A crown or cap is a must since the tooth always tends to blacken after root canal . And the tooth is prone to fracture too .. maybe your tooth has had a vertical fracture or it is reinfected due to improper root canal work .
Next Steps
consult a good experienced dentist who will take some x-rays to find out the cause and do remedial actions .
Health Tips
avoid hard food that side . and seek a dentist immediately . u can also take antibiotics and pain killers for now .