Ideas? Hen with fluid dripping out of vent

When closing up the chickens tonight I found an egg under the roost. It was from one of my two leghorns. (I was surprised that I only found one this morning because they each lay an egg every single day.) Then I put out some scratch and noticed that one of my leghorns was all puffed up on the roost, completely unconcerned at the feeding frenzy going on around her.

Then I noticed that she looked like she was straining and liquid yuck was coming out of her vent......making a small puddle on the ground. Her vent area also looked very large. I first thought egg bound, but both of the leghorns laid perfectly fine eggs today - although one was under the roost which is pretty unusual.

Any ideas? I brought her inside and she kept going after my hand in her usual manner. She ate half of a bowl of food and is acting fine. Her vent has also dried up and isn't so "obvious" now, either.

Thanks in advance for any help.