Ok, that's wrong. Nothing should drink that much.
Is it a wether lamb? Is it peeing ok? Is the belly swollen? (Thinking urinary calculi, maybe it's being fed a cake without the additive and is drinking loads to keep things moving through.)
Otherwise, I'd suspect some sort of infection or blockage or something.
If it were mine I'd check mucus membranes and temperature, if those normal maybe get poo tested for eggs of anything the wormer might not cover, and/or talk to the vet about something broad spectrum.