Yellow stained pool

[Texas Splash]

The funny thing is the pool was crystal clear 2 days ago!!!

You are not alone. Once chlorine meets water that has reached its max threshold for metals, the reaction is quite quick and evident. Many owners are surprised as they say, "We've never had this happen before". But somehow, some way, enough metals built-up over time to cause that reaction. Salt? - maybe, although it seems odd it would be 100% responsible for it. Since you say you haven't used the well, I would consider taking a fresh water sample from the garden hose to the local pool store. While even their metals testing isn't always spot-on, it may shed some light on your situation. You might also Google your country's annual water report. It might surprise you. You can also partially fill a 5-gallon container from your hose and add about 1-2 capfuls of chlorine as a test. See f it reacts.