Penicillin is antibiotic, I think pain relief would be secondary.
Metacam is sometimes used for pain, and there is at least one other painkiller safe for ducks . wish I could recall….
I buy something called Vitamins and Electrolytes Plus . . . don't know if there is anything out your way that is similar. The Plus is probiotics. Feed stores, pet stores may have something.
I am wondering if anyone over on the Cute and Cuddly Call Duck thread may have seen anything like...
I would think starter crumbles would work for them, non-medicated.
Runny poos could indicate infection. It is helpful if you can, to get a veterinarian's help. I am just a duck person, though I have read a bunch here and in books, and have talked to our vet, and dealt with some infections...
Leg weakness, then? Could be a back injury, something affecting her nerves. I just feel I may be missing something that might be obvious if I were right there . . . .
I would get her into a tub of lukewarm water where I could watch her very closely. Check for any sign of back injury...
Can you tell us more? Is there a drake? Has she been laying and then stopped a day or two ago? Diet? Anyone else been caring for her? Free ranging?
The first things that came to my mind, but I have nothing to go on:
egg bound
injured
infection
poison
I would put her in lukewarm water...