Chicken weak and straining

[Newbie to chickens]

This morning one of my lohmann browns has white and pink slimy poop, looks like intestinal lining. She has been inactive and laying down sometimes, straining like she's trying to lay an egg , floofing out her butt feathers, etc.
for some context, all my chickens haven't been laying for weeks due to sudden change in feed (I usually buy feed through a neighbour what I thought was 'layer feed' but was actually grower feed which they were laying so much on). so when I went to buy food from the shop I couldn't find the code for the feed they usually had and had to buy a different one. I've found the code for that feed since and am buying it myself today.
This chicken has access to calcium but doesn't want it. Is drinking and eating as usual. Please help.

 

[Wyorp Rock]

Do you have photos of her and her poop?

If she's straining and acting like she's trying to lay an egg, I'd give her Extra Calcium asap.
300mg Calcium Citrate+D3 or TUMS now, then once daily for a week.
Pry her beak open and put the tablet directly into the beak and let her swallow.

I'd also mix up a little sugar water and give that to her to see if I could get her to perk up. 1tsp sugar to 1cup water.