Dogs love bread, and Labs are often very food driven, so this may have been more than she could resist. The best way to keep her safe going forward is to kennel her. Her loose stools are likely due to eating unusual food, and the irritant effects of a plastic bag. If the wrapper wads up though it can get stuck and lead to an obstruction, so hopefully she ripped it up as she ate it. If she starts vomiting, has a tense painful belly or refuses to eat or runs a fever (more than 103F rectally) then that can signify a blockage and she should be seen by a veterinarian immediately. Surgery may be needed at that point. The wrapper pieces may take a couple days to pass, keep checking her stools. I would expect that she can pass them but you'll just need to watch her closely and hope she ripped them up a little.
In the meantime to help with her loose stools and until the pieces pass, she'll need to be fed a bland diet
with lots of fiber added in small meals several times a day, 4 to 6 meals is ideal. We do this so the pieces are surrounded, and the gut isn't irritated contracting on plastic, and they don't bunch up and cause an obstruction. A homemade diet for this is 1/3 boiled, lean hamburger or boiled, white, skinless chicken
, all fats drained off the meat, and 2/3 boiled, white rice. Add 1-2 tablespoons of canned pumpkin, (not pie filling, just pumpkin), to each meal for fiber to help cushion and push through the wrappers. This diet will also help with her diarrhea. If you aren't seeing fragments of the wrapper in the next day or two a veterinary visit is an option. If it doubt at all that things aren't going well it is always best to have her checked. Once the pieces are passed and she's feeling well then start mixing in her regular food, adding a little more regular and less bland at each meal. It should take a week to get her converted back to regular food.