Aha - I'll bet the tree IS the culprit!! Just did a little research and found out that the tree oil can irritate their stomachs if they drink the water - and they always get in there and drink the water. I never knew this before and it was never a problem in the past. But this particular species of tree we got this year has a very strong scent, so I guess the oil is stronger?? I think it's time that we buy an artificial tree going forward.