Passing only white poop and clucking in pain

[The Chickens' Maid]

I have a 9 or so year old Jersey Black Giant hen who is passing only white poop, which I think is urates, and is clucking in distress every time she does. This started last night, but she's had an ongoing problem with mites for a while now, which I'm wondering if it's related (every time I get rid of them, they come back. There's been a very resistant strain this year, but the other chickens aren't getting them to the same extent as this hen.) She likes to sit outside in the dirt, I think it's to cool herself. She will walk around, but she walks as if her vent/back end is very uncomfortable. I washed her back end off last night to help get rid of the mites, which seemed to help a lot. She really liked the water on her vent. If anyone has seen this sort of thing before, I would really appreciate some advice. I am getting so tired of this. All my girls are old now, which means they are all dying of one thing or another. She's the second one this year.
Thanks!

 

[The Chickens' Maid]

Update: she's still hanging on. She seems better when she's not trying to pass something. She seems to want water and is trying to cool her abdomen down a bit. I gave her some ice in a towel to sit on, which she did for a second, then got up.

I have some broad-spectrum chicken antibiotic that expired in 2016. Should I use that?