It's really hard to tell what is wrong with birds sometimes. Diarhea is a common symptom in many diseases. Are you in a hot climate and the bird may be suffering from heat stress? Make sure it has shade and fresh water if that is the case.
The best thing I can tell you is if you have a dog crate or small pen to put her in, I would do that for a few days. That way, you can monitor how much she eats and drinks and what her poop is like. It could be she has a blockage and it self medicating by drinking alot of water, but it could very well likely be that she is dehydrating because of the watery poop and trying to keep herself hydrated. So, I would pen her and see if she is eating food. Then I would remove her food and water at night and first thing in the morning see if her crop went down to normal size. If it did go down, and she is still pooping, she is not likely blocked. If she still has a huge crop, not much to do but give her some vitamins and minerals in her water and let her eat and drink what she can. Soft food would help instead of pellets or crumbles. Gently massaging her crop a few times a day could help break things up if the blockage is in the crop. Sometimes nothing but surgery will help if it is located in the crop.
There are also some diseases out there that cause a bird to drink more and I think fowl typhoid is one. Diarhea would be yellow, but she wouldn't be too interested in food and would probably be inactive. Diarhea is a common symptom in many ailments. If she is molting she won't be laying for awhile, so if it was me, I would try some antibiotics in her water that treat for an e-coli infection. That's the first thing I would try. You can check the feed stores or online for medications. They are not approved for layers, fyi, so that's your call. They don't give a withdrawal time for eggs generally.
Her internal flora might be messed up also for whatever reason. You could try some probiotics in her feed or water for that. Or she could have a yeast infection in the crop if it is smelly. Open her mouth and see if her breath smells like yeasty bread. Sometimes that is a result of a blockage because feed ferments in the slow crop. I believe you can find remedies for treating that by searching "sour crop" on this forum. If you do medicate her, make sure you do give her probiotics after finishing the antibiotics.
