Hey buddy. I think it's a calcium deficiency. I had the same problem with my Quick Dinamed CBD, it showed up at the same stage and with extremely similar spots (check out my Quick Dinamed, you'll see what I mean). I made the mistake of thinking it was due to a phosphorus deficiency, which gave the calcium deficiency time to spread. You should add Calmag to the next watering, or you'll see all its leaves necrosing with light rust spots and then grayish ones, and it will happen fast.
I hope this helps you.