If they have come from a field with nit much grass and they have just been separated from the ewes/dams, then yes I would say it is probably the grass plus stress. I would get them inside overnight and feed hay to help them dry up, fill them up with hay in the morning and then let out for shorter times until they are more used to it.
If they are listless and not looking right, then I would also check their temperature, pneumonia can also be accompanied by diarrhoea - I have found this to be the case in my goats (both youngstock and adults). If they have a temperature, then a shot of Alamycin LA (antibiotic) may be necessary.