Stomach distended and in pain sounds like bloat. It's a bit early in the year as normal cause is eating very rich, clover pasture.
However, it can be caused by gorging on unsuitable food. Is this possible?
If rumen is just distended, not completely blown up, then if you can get some vegetable oil or liquid paraffin down, it should help.
However, if the rumen is REALLY taut, like it couldn't stretch any more, then it is full of gas,it will press on the heart if untreated and the animal will die.
In this case you have to release the pressure as soon as possible - not a job for the squeamish - BUT IT WORKS and the animal will die if you don't. So you've nothing to lose.
SO - get a sharp (clean!) thin knife and stab the left hand side of the sheep, just backwards of the last rib. You should get a sudden release of gas like a ballon going down and sheep should instantly feel better. The wound will seal itelf.
Do it purposefully - direct the knife straight in, = you won't go too far as you'll only be going deeper into the rumen (that is, the large stomach which takes over most of the left side of the abdominal cavity.)
Anyway, hope you've sorted your lamb out by now. You probably won't have to do this, but it's always useful to know.
Good luck. Let us know how your lamb goes on.