I had a welsummer that started laying those every time she laid an egg. After 6-8 months she stopped laying altogether. If you can zero in on which chicken it is, you may check on this.
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I haven't found another one like that yet (or heard from somebody that they did) so I'm hoping it was a one time thing...I just wish I knew what chicken it came from!
try isolating one chicken at a time to check which one lays bloody stuff. Can also check the chickens for signs of inflammation blood on the feathers and other abnormalities.