Did you dry the ewes up properly last year at weaning time?  We cut the feed way back about a week before we pull the lambs, only give them hay, no alfalfa or grain, and then dry lot the ewes on poor hay for 10 days to 2 weeks after lambs are removed.

The other thing is it could be OPP.  (ovine progressive Pneumonia)  That is picked up by the lambs at birth and can show up anytime after age 3.  Usually one side gets hard the first year and both sides the next year.  You can test for it to see if you have it, but it's not practical to eliminate it from the flock by testing.  The only solution is cull hard and keep NO lambs back from any mother that has a bad bag.

If they weren't nursing on side last year it was because it has no milk on that side.  My money is on OPP