Place chicken breast in a crockpot. Cover with BBQ sauce. Cook on low for 8 hours or high for 4 hours. Remove chicken and shred. Return chicken to the crockpot and coat with BBQ. Divide BBQ chicken between two tortilla shells. Top with shredded cheddar cheese. Put a second shell on top. Heat oil in a skillet over medium heat for 3-4 minutes on each side until golden brown. Serve with salsa and sour cream, if desired.