If you haven't been feeding the ewe introduce 18% protein ewe feed pellets slowly, to give the rumen time to adjust. Start with 50g twice a day and increase to 500g in 50 g/day increments then reduce back down again after about 3/4 weeks, depending on her condition and your grazing. This will help milk production and keep her in reasonable condition. Had you vaccinated her for clostridial diseases (usually done 4 weeks before lambing)? I'd watch the other ewe like a hawk - what happened to one may have happened to the other. Signs of imminent lambing include pawing the ground, going off into a corner or under a hedge, squatting without urinating or defacating, lying down with legs outstretched and clearly straining - all of these may happen before or after you see a waterbag appearing, or you may not see it at all.