Was it Cocci?

I have six 20-week-ish old Barred Rock pullets - raised them from day-olds and waiting on eggs any day now.

About two weeks ago I noticed some bloody droppings. They seemed to be flock-wide and lasted about three days. Everyone was acting normally so I didn't worry too much about it. Cue googling and chicken hypochondria. Every picture / thread / random article I could find pointed to Cocci, including The Poo Chart (http://chat.allotment-garden.org/index.php?topic=17568.0). The droppings were full of what looked like dark red blood, but weren't watery. Not intestinal lining strands.

Two weeks later, all my chickens are still alive and well, and I'm stumped. I did not treat for Cocci - by the time I figured out what it might be, the blood was gone and poops were back to normal. I gave a probiotic / electrolyte supplement for two days just in case. From everything I read, Cocci advanced enough to produce the lesions that cause bloody droppings is pretty deadly. No one died, or was lethargic or puffed or anything else associated with Cocci. (I have Corid on hand just in case.)

My chickens are still on chick starter - I feed Nutrena Naturewise. They eat food scraps and have access to pasture. I usually let them free range for a few hours a day, where they have had access to blackberries and those red bear/bird berries that are all over upstate New York. I had recently switched from chick grit to adult grit. They have not started laying.

Any ideas what could have caused dark red bloody poops? Some berries they ate? Too-sharp grit? Alien monsters invading their intestines? Any suggestions would be great!!

Thanks so much!