You can try Penicillin or Amoxicillin as @azygous suggested.
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-Lethargic
-Crop was hot and swollen yesterday, less full but very saggy today
-Still drinking a ton of water
-Not eating pellets but ate some cracked corn and mealworms
-yellow runny poop that looks like egg drop soup
We need to go back to the original symptoms, minus the OP's attempt to diagnose. Those also can be symptoms of pendulous crop.
Other mentions of "saggy" and "water balloon" and "drinking a ton of water" are giving me an "ah-haa" moment. Pendulous crop can cause the contents of the crop not to empty completely, eventually resulting in a chicken that feels ill, loses her appetite, loses weight, etc. And drinking "a ton of water" is a desperate attempt to flush the contents of the crop out.
If the Thelma's crop, and older hens are a candidate for this, hangs down low and swings back and forth when she walks, she has pendulous crop. She could recover her health if the crop were to be gently supported so the contents could naturally empty. http://www.the-chicken-chick.com/2016/02/pendulous-crop-in-backyard-chickens.html Try lifting the crop and supporting it in your hand while massaging the contents in an upward direction. The crop should empty when you do this.
The solution is to rig up a gentle support. There is an item called a "crop bra" that MyPetChicken sells if you don't feel competent to rig one up yourself.
What ails Thelma may not be EYP at all, but pendulous crop.