look up sour crop or slow crop.
I have a girl with this issue and if left untreated it can be deadly. Its good that its still squishy and she's still active. From what I understand, its full and squishy because of all the gases that are building up in there as the food that is stuck ferments.
To help, seperate her and do not feed her for 24 hours. give her water mixed with apple cider vinegar - 2 tbls per gallon (the organic kind with "the mother" - I use Braggs and you can buy it at Stop & Shop in organic isle) that should combat the bacteria and help clear out her system. some people also vomit their birds however for the unexperienced, you could potentially "drown" her.
This morning I gave my girls crop a good massage and she vomited herself twice. I then put her in a dog crate (in the coop so she's still amongst the other girls) and gave her a little yogurt to peck at. Good probiotics!
Hope this helps and good luck!
Edited to add - only need to do this if the crop is not empty in the morning (hense the 24 hr no food thing).