I am so sorry to hear about your goldfish. Losing a pet, especially a young one, is heartbreaking.
It is impossible to know for sure without more information, but here are some possibilities:
* Overfeeding: Goldfish are notorious overeaters. Too much food can cause fatal bloat.
* Bloat: Even without overfeeding, bloat can result from stress or diet changes.
* Internal parasites: Parasites can disrupt digestion and lead to death.
* Water quality issues: Goldfish are sensitive to ammonia and nitrite changes.
* Sudden illness: Fish can become ill very quickly.
What You Can Do
* Review your feeding routine: How much and how often were you feeding?
* Check your water quality: Test for ammonia, nitrite, and nitrates. Perform a water change if levels are high.
* Consider a fish vet: A specialist can help determine what happened and how to prevent it.
I know this is difficult. Fish can be delicate, and sometimes things happen outside our control. Do your best to learn and keep your remaining goldfish healthy and happy.