Look bro nobody here KNOWS what the problem is, can only speculate. Your the one whos been messing with it the whole time, your going to know if you've been over watering, over feeding, ect. Several things can cause deficiencies or the appearance of them. Prime example my plant looked like it had a Cal/mg deficiency at first, which evolved into a phosphorus deficiency, the obviously remedies were not working to fix the problem. Turned out the water i was using was "hard" and had excessive amounts of calcium and mg already in it, which caused my soil to lock out the phosphorus, so it was'nt really a deficiency at all. Its things like that you need to consider. Your best bet would be to flush the soil well, then on next feeding give it the normal dose it requires. If your over feeding you will notice things will keep getting worse, if your under feeding you will notice the deficiency progression slow down over then next week but not correct itself. I have a feeling you may be having something cause lockout of another nutrient. I dont think its over feeding because a key thing to remember is, nutrient burn or over feeding of something will show in the new upper growth first, where a deficiency will appear in the lower older foliage first, like yours..which means the plant is'nt getting certain nutes from the soil so its taking it from the older bigger leaves
