Are you sure it was plastic? If it has come out (whatever it is) you will be fine, your body is flushing it out.
You mean it's the cello wrapper of the o.b.? That is certainly possible, and it does not necessarily mean the o.b. itself is still in your body. But you could hop over to your doctor for a lost-tampon check. It happens a lot, and your doc will not be alarmed or puzzled if you tell him or her that this is your problem. It would be worse to leave it in, they do get stinky (like old meat) and can cause an infection.