Sometimes they pass a bit of the membrane from their oviduct or intestines in with their poo, and this may appear to have a little blood in it, but this is harmless. Occasionally they pass what looks like a largish chunk of meat, like a skinned sausage,again a piece of discarded intestine, this is known as a lash. People can get very alarmed at the sight of this but it's normally quite harmless. Chicken innards are very different to ours, rather complicated, and the oviduct isn't fully separate from the rest of the birds intestines. Sometimes this can cause peritonitis when a half-formed egg goes down into the gut instead of out the normal exit, and this gets infected and is fatal. This is most common in older hens who have laid many eggs and their innards have got slack. Not anything you need to worry about with your young girls!
This is an interesting article about egg abnormalities - a bit graphic though, so be warned! . http://www.aviculture-europe.nl/nummers/07E03A06.pdf
And this one is about lashes http://www.backyardchickens.com/t/473989/weird-egg-weird-clump-lash-graphic-pics
-just so you don't faint if you ever see one!