Swayback can appear later, having been unnoticeable at birth. As the lamb grows, there's more weight for the skeletal system to carry, and any weakness becomes more noticeable.
However, your description does not say 'swayback' to me. If it's not been hit by a car, or another sheep, I'd be inclined to support the spinal abcess theory. Often these do die, but not all. If the lamb is otherwise happy, and eating, drinking, pooing and weeing, then yes, steroids, antibiotics, anti-inflammatories, and see what the vet says.
Just realised I'm pontificating when we've already had a vet on.

I'll shut up, I've nothing to add! lol