What could have been the cause of her death?

My budgie died this morning :'( she was about three years old, and i read somewhere that they live up to ten years old, so i thought, that's really weird. I had two hyperactive budgies this morning, and i noticed they were being a little quiet than usual, and that i heard like this squeaking sound, like heavy breathing. I went to check on them in panic, and i saw her on the bottom of the cage with her wings outstretched and looking like they were cramped. She was breathing really heavily, and her eyes were opening and closing slowly. When my mom tried to pick her up so we could put her into her little box to take her to the vet, she kind of teetered and tried to regain her balance, all in a cramped motion. My mom decided it's better if we left her in that cage just in case, and i went into the other room. My mom then came out and told me she died, and that she thinks it might have been a heart attack. One of the perches was diagonally, as if it slid off the bars, and i thought maybe she freaked out so much when she fell off that she started having a heart attack. It happened many times that those two were bickering, and she lost her balance and fell off, but she just jumped back up and kept screaming at him. She does seem to have been sickly lately, active as always but her beak and nose were very dry. when she had a long strain of flying, she had kind of a squeaky breathing, but was fine soon. I'm really sad and i would really like to know what happened. Pls answer....