pH maybe?

I can't tell which leaf that is... a new one? an old one? If it's only one leaf and it doesn't spread, I wouldn't worry too much about it, especially if it's one of the first leaves the plant grew... And no, you haven't started your nutes too early - probably just about right! Don't worry that this plant isn't performing the way others have... each strain is different and even each seed within the same strain can be different. It sounds to me like you are really "reading" your plants and doing a good job of it! I will tell you, though, that I fall into the same trap thinking "oh, these plants are SO tiny, they will never get as big as (fill in the blank)" ... Don't worry - you're doing really well! Good luck!