2022 Update, it happened again, I think my pool winter cover is to blame, I think its kind of porous, water (filled with dirt/alge/leaf gunk) is probably constantly going through the cover, had a like black film on the whole bottom. This year I was ready for the possibility since I suspected the cover needs to be replaced. It was a cheap warranty replacement which wasn't as thick as the one I originally bought which I complained to the company but I had no choice but to keep it 3 years ago....
Anyhow, I ran my filter for 1 full day before uncovering, Uncovered in the evening, vacuumed and brushed. Tested the water. Added half a gallon of 10% pool chlorine and the chlorine was eaten in minutes. Ran to home depot, got 4 gallons of 10% pool chlorine, plus 1lb of powdered shock that I had left over (not buying it anymore) , and started my overnight SLAM. My goal was to hit it hard before sunlight touches the pool and makes the problem worse, finally after gallon 3 I went to bed at 10:30pm, ran the pump all night, and woke up to a chlorine level of 3 ppm, with 0 CC, so at 5am I figured id add the last gallon of 10% chlorine just in case I didn't test it overnight but I just went overkill on the chlorine in hopes it would work, I'm not home to keep up on it so I'll see if I need to continue to SLAM tonight, but at least I'm in much better shape than I was last evening.
I still cannot believe how much Chlorine I need to use, never in my life have I had to open a pool and dump 4 gallons of 10% pool chlorine so I'm positive I need a new winter cover now, the fact that it ate the chlorine so fast is still so shocking to me but at least it makes sense now after the second time going through this, hoping I come home and I still have like 3-5ppm FC if not I'll buy more pool chlorine and work harder to maintain SLAM levels.
Thank you again for all your help last year, now I am more knowledgeable!