Oh dear, sorry to hear about this.
Sounds like she didn't cleanse properly. Antibitiotics and metacam plus palliative care is the norm for that. I have one on day 2 of treatment for this just now, although thankfully she has passed most of the afterbirth now, and is still eating a little, feeding her lamb and moving around, and has no diarrhoea. Our vet has just explained that the retained placenta does in fact need bacteria to help it come away, but that the trick is to stop the infection crossing into the bloodstream. We're using Pen & Strep which he says is a good one to use in the circs, and to keep giving it until she is right, and to give her Metacam every other day until she seems comfortable. And that there is little else they can do; she needs time, and TLC, and to be sure she doesn't get dehydrated and to try to tempt her to eat if she isn't eating.
You may need to rehydrate your ewe if she has diarrhoea and isn't drinking : Rehydion or PFS or similar. It will give her some sustenance too.
Sometimes poorly ruminants appreciate a bit of ivy, so if she doesn't have any in reach it might be worth putting a bit in front of her. Don't force it on her though; she will know if she needs it and it can be toxic in the wrong circs.
Hope your ewe improves. If you think she is in pain then more Metacam would help with that. Do you know what antibiotics she had and when was the last dose?