The udders don't shrink to nothing, they just stop being big and turgid. By the time you are checking your girls before tupping, they'll be smaller and more floppy, but not completely flat.
I'd do as Melmarsh suggests, let them onto the rest of the grass steadily and keep a watching brief. If it's clover-rich it might stimulate production, otherwise, after 3 weeks without their lambs, they should be fine.