In a medium bowl, stir together BBQ sauce, ground mustard and brown sugar.
Place pork loin in the slow cooker. Season with salt and pepper. Pour sauce over pork and cook on high for 4 hours or low for 6-8 hours.
While BBQ is cooking, stir together slaw mix and apples. Mix together Greek yogurt, vinegar, oil, celery seed, sugar, salt and pepper in a bowl. Pour over slaw and combine until covered.
Shred pork once finished cooking. Serve in tortillas (best when warmed) and top with slaw.