Our Guinea Pig Lost A Lot Of Weight In 5days

Our guinea pig lost a lot of weight in 5days when boarded at a vet. On return he ate well and ran well for two more weeks. One night he was alright the next morning dragging back legs. He became weak and then muscles in mouth appeared to tighten. Rushed to vets. His temperature low and breathing low. The vet put him to sleep. Was he sick or did he get trauma from a mouse we had in the house? The vet said he may have had cancer but I didn't mention the mouse.