Blood on eggs

[Ditsy4]

Hi,
I'm new to keeping chickens and I came home from work today to find two eggs with blood on. What is the reason(other than difficulty shifting egg) and is it anything to worry about?
I have three ex battery hens who are penned till I get home then allowed in the garden. For a couple of hours a day. Fed organic pellets and a corn mix.

We have had this over the last two days as well, from the same chicken. The second egg was quite a large one for the chicken in question, but the first one was small. On the second day she went in and out of the best box 2-3 times to attempt to lay it. Currently waiting for today's egg. Sorry I can't help but watching with interest in case anyone else has experienced this.

It hasn't happened again. We had a tiny egg the other day though.