Sounds like pasteurella to me too. There are various strains so vaccination may not rule out all of them - we use Ovivac P Plus. Sudden deaths in otherwise healthy lambs around this time of year are generally put down to one of these. Can cause a massive septicaemia - first symptom is sudden death, alas.
I use Ovipast and Lamivac to cover all bases for the lambs and the goats at the same time, then move all adult sheep onto heptavac - it works for us because of the goats.