Ok, it could be the start of water belly. I'm not too familiar with sour crop. Here, I'm tagging the person who helped me diagnose my first hen that had water belly, if it's water belly, perhaps she can help you more, and- if it's not water belly, she can help you know what it is. :) @Eggcessive
Ok, my second hen I didn't see a waddle. When you pick her up, does she appear to be heavier than usual? If you don't know her weight anyways, does she just plain feel heavy? When she eats from the ground, does she try to keep her body straight, so therefore stretching her neck further just to eat?
The belly will look slightly blouted and red. Yes, it can cause the poopy butt. I've had two hens who had water belly, and they both got it at about that age. The second one I saw puke out yellow stuff that looked like egg yoke. Gently place her on her back and feel her belly and take a picture...