I'd suspect either poisoning or sour crop.
But here's something to help you with the others. It's what I've done for years (once I learned the importance of doing it) and it's a wonderful way of heading off problems before they happen. It'll help you save the rest of your birds.
First, weekly, pick up all birds and check them over carefully. I'm sure that Mr. Jorday would help with this.
You'll want to check carefully against the skin, especially around the warm moist area of the vent, for parasites. Look carefully for eggs, the bugs themselves, etc. I find them more often on the feather shafts than the skin - except around the vent.
While you're looking at the vent, make sure it's relatively clean - no black or white waxy stuff, no redness'/swelling/irritation, not many droppings around those feathers.
Then feel the abdomen very delicately. Check for excessive weight in the back part of the abdomen (which can indicate internal laying). Check the "keel score", the sharpness of the keel bone running from the chest to the abdomen. You should be able to slightly feel the keel bone, but there should be rounded meat on either side - quite like the keel of a boat. If it feels like a spatula, the bird is underweight.
Listen to their breathing briefly - watch if they're breathing is labored. In upper respiratory problems, labored breathing is open mouthed. In lower respiratory, it's tail bobbing. This doesn't include summertime panting (often accompanied by lowered wings and a look that says "I could use a cold drink and a nice air conditioning").
Feel the front of them where their crop is. keeping in mind when they last ate, determine if it feels normal. Normal (after eating) can include hard-as-a-rock-like-an-overstuffed-beanbag. In the summer, it's not uncommon for their crop to be filled with water. it usually should not be mushy, and it should never smell terribly bad. (Chicken breath is weird at best, but shouldn't smell sour or horrible.) If you're concerned, go back when they're asleep before they wake up in the morning and check again. The crop should empty in 24 hours. Night time gives the crop a chance to empty. If it's still full in the morning before they get off their roost, their crop is problematic.
In your case, I wonder about sour crop.
Generally give them the once-over and pet them. It'll make it easier to handle them when they're ill, and this way you can catch many illnesses before they get too bad.
When you put them down, check their droppings to make sure all are normal. There should be mostly solid droppings with white urates on top, with the occassional bad-chocolate-pudding cecal clearing dropping amongst them.
I'd do this for your other gals and let us know if you find anything abnormal.
By the way, i'm very sorry for the loss of your girl. Barred Rocks are especially dear to my heart as they're my absolute favorite chicken. My condolences to you and your wonderful son.
