Certainly make sure there is plenty of fresh drinking water available at all times. Offering a bottle of slightly warmed water between milk feeds may help, as you say she is peeing no problem.
Castor oil is fine, any kind of vegetabe oil, or paraffin, to help unbung a constipation. You may need to give quite a bit...
Do you see the lamb cudding ok? If its rumen has stopped working you could give natural (live) yoghurt to help restart it; some people mix a little of this in with the bottle milk as a routine thing.
A little sodium bicarb can help restart the rumen too - be aware that it can cause some spectacular belching and farting!
