Sometimes, a blood vessel ruptures in a hen's oviduct or a piece of tissue gets lodged in the egg, and causes this. Its just a fluke ocurrence that happens every so often. It may be unappetizing, but doesn't mean that your hen is sick.
You won't see blood in a fertile egg until it has been incubated for several days. There's only one subtle difference between a fertile fresh egg and an infertile fresh egg. A fertile egg will have a tiny white spot that forms a bull's-eye on the yolk (kind of like a doughnut). Infertile eggs will have a white spot too, but it will be poorly formed, without a hole in the middle.