Have you had a look around to see if there is anything dangerous that has been dropped, spilt, left open anywhere where your dogs have access? Be it accidental or intentional?
We woke up one morning about 18 years ago to find both of our youngish cats dragging themselves along the paving stones to the back door retching and crying. We didn't know what had happened for 3 or 4 days but eventually we found that they had somehow come across some antifreeze. Logic would say that they'd have possibly licked it once and then said nuh-uh, but whatever they did or didn't do, they both ended up having to be put down after a few days
taught us a lesson about keeping chemicals out of harms way.