I'm off to see the vet this morning. I haven't a clue what has caused this.
The field has been kept empty for weeks to help ensure it was 'fresh', likewise, their shelter had fresh bedding, is bright, airy, not draughty.
I always give colostrum at the same time as doing the lambs navels with purple spray, and it was a new bottle purchased from the vet that morning. As was the milk (which I always get in at lambing time just in case needed).
I watched to make sure mum was ok with the lambs and watched the lambs take to the teat ok, and checked the milk was flowing.