Have a very similar ewe and similar problem. Birthed herself, lamb and ewe ok for about 12-24 hours and then ewe looking miserable, head down, minimal milk and scouring.
Vet checked I hadn't missed a second lamb (I hadn't) and gave her painkillers and antibiotics. I wormed and flaked her as I would with my ewes after birthing and before i turn them out. She's had her second dose of antibiotics and is starting to pick up although not herself yet. Not sure her milk is going to come in enough for the lamb but he is happy to take what he can from her and the rest from me. Get your vet in.