Every animal "goes stiff" shortly after dying. It's call rigamortis. It doesn't last long (a few hours in some animals up to 12 hours in others depending on size and environmental conditions).
If she was stiff when you found her she most likely died within the past six to eight hours. Rigamortis sets in fairly quickly after death. If the other animals you've found in the past have been limp it was because rigamortis had already completed it's "cycle".
From the symptoms you've posted it would be difficult to determine what actually caused her death. If you really want to know I suggest bringing her to a vet to do a necroscopy (don't freeze her in the meantime - freezing can destroy some of the "evidence" the vet will be looking for).