ok can anyone shed any light on this?

My Mother in Law had a bout of diarreoa last week, which took about 4 days to get better. then she passed through her bowel something really disgusting, she kept it to take to Docs. It looked like the shape and size of an index finger but it resembled a king prawn shell case !! she HAd eaten king prawns 2 days before and im wondering if she maybe ate the shell, but im sure she wouldnt.

The Doc has sent off the specimen, but im wondering what the heck it could be.