Was Gunshot Sam an older dog? Vet told my niece that Chihuahuas are prone to heart problems as they age. Hers is 8 yrs and already has to take daily medication.
I am so sorry about the sudden loss of your little Sam.
It is hard enough to lose a dog you love when you are prepared for the inevitable. But when it happens like this, there is so much shock, and it's hard to understand or cope with. Especially as he seemed to be recovering just fine.
Poor little guy. What could possibly have happened is a clot broke away from the injury, and travelled to his heart, or brain, causing a sudden fatal heart attack or stroke. This can happen, though it is quite rare, it can still happen.
Please do know that he is at peace now, knows you love him. Love never dies. Bless his Soul.