This happens every so often in the Spring with some of my hens, too, when their egg laying apparatus is gearing up. Now is their big push to lay eggs, holler about it and go broody. It doesn't necessarily mean they are ill. It's also not unheard of for a hen, especially a young one, to lay an egg followed by another the next day, so I suppose it's possible for her to lay two in one day...are you positive it was her?

Watch them closely for any signs of illness--listlessness, not eating, drinking or carrying on normally, losing weight, etc. Check their bottoms for soiled or bloody feathers, and make sure they aren't bleeding from their vents. If an egg has rough calcium deposits on the outside, it might have torn them a bit on the way out (this happened to one of my hens about a week ago, now she's fine). Check their poops for worms and their feathers for mites, and make sure their water and food is fresh and not musty or moldy--same for their foraging area. Nesting material needs to be fresh and dry.

If they look healthy and carry on normally, I'd say watch and wait. Check for any more bloody eggs, and bloody bottoms. If they are bleeding from their vent, isolate them to keep other birds from pecking at the blood, treat the bleeding with Kwik-Stop (a powdered styptic, get it at the pet store) or corn starch will staunch bleeding in a pinch.

I've got a chicken info site here that may help more, here is the page on 'Health':

http://jackshenhouse.com/VSChickHealth.htm

Velvet ~:>

I am getting a bloody egg every few days now and have a hen with a bloody vent. What causes the damage to the vent? Why is it bloody?

I know the other chickens peck at the bloody vents but why was it bloody in the first place? The eggs with blood on them are smooth and normal.

Here is a link that might be useful: My Country DIY Blog

I think that Velvet gave some excellent advise. The only other possibility that I might add is if they or she could be dehydrated? This will cause blood on eggs.

I found a bloody egg this morning. I checked the girls bottoms for blood and none showed any. The egg was a normal size, not a large one.
Luke O said dehydration could also cause bloody eggs. It is freezing here and the girls go several hours, several times in 24 hours with frozen drinking water. I replace with fresh several times a day. Maybe they are slightly dehydrated. Also I've been giving them salad mix to suplement the feed for the lack of plants this time of year. They only go into the yrd about an hour a few times a week. It's been -0 to 20 lately. I try to let them out when it's 20ish and sunny.
Any ideas on the cause of the blood?

A few thoughts...

Was the blood very red and fresh-looking, or brownish and clotted? Fresh blood would indicate it was deposited on the egg lower down in her tract, just prior to being laid and nearer her vent, while older, brownish blood would indicate it happened higher up in her tract. A previous egg could have broken inside her and the shell edges could have cut her, or possibly it was a mating injury if you have a rooster.

Was she a young hen, just coming into lay? Sometimes this happens to girls who are just starting out.

If you live in an area where the frozen water is a yearly issue, you might want to invest in a heated water dish. It would save you lots of toing and froing to change their water numerous times a day. :)

Velvet ~:>