a few of my chickens are pooping something that looks like mustard

[Brooksy21]

This thread is old - originally started in 2011, then revived in 2018. Your posing a question, so I assume you need some help?

What do you feed, how old is she, when is the last time she laid an egg?
The urates in the poop photo you posted are yellow. This could indicate something going on with the liver. Fatty Liver Disease, reproductive disorders, cancer, etc... sometimes can cause that.

You mention lethargy - consider getting a fecal float to see if worms may be part of the problem, feel her abdomen for bloat or fluid, look her over for lice/mites, make sure her crop is emptying overnight.
Work on hydrating her, then see if she will eat/drink. There's numerous issues this could be. Hopefully with a bit more info we can offer better suggestions.

Also, I feed them a mix of laying feed Pellets with grower crumbles, trying to use it up (the grower crumbles), I also add some oats and sunflower seeds here and there, with grit and oyster shells. I do treat them with Chicken Scratch and/or meal worms couple times a day. I probably give them more scratch (cracked corn/wheat) than I should.