Problem 3rd week flowering sour diesel

I have no idea what's wrong. This is only my second grow. I have one plant sour diesel under 400w hps in an approx 10 gallon planter filled with miracle grow overwatering protection soil (not the best, I know). This is the second grow with this soil so I assume a lot of the prestocked nutes are gone. I'm using fox farms grow big every other water. The plant started yellowing at the bottom leaves, tips inwards and the problem is steadily rising up the plant. I checked ph, which was fine. I doses it with a little more grow big than usual, suspecting a nitrogen deficiency, to no avail. I'm think inking maybe over-watering? I really am at a loss and was hoping someone here could help. I also noticed some brown spots on a leaf and thought perhaps it's mold so I put a dehumidifier in and have it around 40% at the moment. Any ideas gentlemen? Any help would be greatly appreciated.