Hubby spoke to the vet a little while ago. Lily is going in for exploratory surgery. Poor little (well, not-so-little) mutt. The vet said her spleen felt enlarged and also wants to make sure she doesn't have an intestinal blockage left over from last week's toy eating. As far as the spleen goes, he said it could have twisted and interrupted the blood supply, or she could have a growth of some sort. He doesn't feel that cancer is likely since she's in good health and is on the heavy side.
We're keeping fingers and paws crossed that all will be OK.