I have had birds (other types just getting into quail) look healthy and do that. Everything from chickens to parrots. Just hearing the evidence I would say her heart have out cause of her age. Looking at the facts.
She was in a secure place with no evidence of foul play ( chicks still in good...
Their poor little bodies wear out quickly. The average life expectancy is three years. Some live longer. It may have just been her time to go. She had a long and it sounds like happy life.(autocorrect hates me)