So sorry about your hens.
If your hens have had bloody DROPPINGS along with bloody vents, this could be coccidiosis.
If they are having trouble passing eggs, they could be eggbound. Sounds like this is indeed the problem with your current hen, although there could be other factors involved.
For the short term, I would recommend a nice warm "sitz bath" - - if you have an old dishpan, or if you dearly love your gal and are willing to let her occupy your sink, run nice warm water in it -- enough to cover her bottom -- and gently hold her in it for 5 minutes or so. This can help relax the vent and help her pass the egg. [be sure to dry her off well afterward.]
What are you feeding your chickens?
What is your weather?
Are there other symptoms? Are they pecking each other's bottoms?
Hope that helps as a start. Good luck, and please keep us posted.