As long as it's a small piece of plastic, I doubt your cat will have any problems digestively. That's where the dangers of strings and other odd objects, which once they become entangled, can kill your kitty. My own cat had rubber band fixations, and it was evident to me seeing as her pooping rituals and schedules weren't normal (being attentive helps).
Finding the stupid rubber band in her feces further confirmed my suspicions, hereby banning such things and plastics from her furry reach. To reassure you though, if my cat can be fine after eating a rubber band or two (or three+, knowing her...) I'm sure your kitty will be fine if it's just a piece of plastic. If you want to just be safe though, consider changing her food to something like nutro's indoor cat formula that targets good digestion (among other things like less smelly poop). Good luck!