It sounds to me like she has an acidy tummy, eating grass is a way they make themselves sick, though it doesn't always mean they are ill, but the fact she is enjoying milk points towards a gassy upset tummy, Elspeth try and get some natural live yogurt for her, a Bio pot or something like it, I also give mine buscopan if they have upset tummy's and crampy tummy's. For Ceilidh I would start of with the Live yogurt it helps balance the stomachs natural enzymes and bacteria.
Keep me posted.
