Preheat grill for medium heat and lightly oil the grate.
Grill chicken breast halves until they show dark grill marks and are no longer pink inside, about 5 minutes per side. Let the chicken breasts cool, then place into a bowl and coarsely shred with 2 forks.
Bring a large pot of lightly salted water to a boil. Cook orecchiette pasta in the boiling water, stirring occasionally until cooked through but firm to the bite, 5 to 8 minutes. Drain and return pasta to pot.
Stir chicken breast meat, pasta sauce, broccolini, stewed tomatoes, portobello mushrooms, garlic, and red pepper flakes into the cooked pasta. Place over medium heat and cook, stirring frequently, until the mushrooms and broccolini are cooked through and tender, 5 to 10 minutes.