Thats exactly what he hadI'm very sorry for your loss. Its hard when they go. You tried everything you could though, and it's not your fault.
It sounds like little Poppy had pneumonia... I delt with a lamb who had it not too long ago. Hers was from fluid in the lungs from birth, and they did treat her with antibiotics. The only advice I can give is if you see anything like it again call the vet and take the lamb in asap. Head bent low, drooling, everything you said resembled the lamb I helped.