Deficiency week 2 not sure why

Well, I don't know if it's a calmag situation, quite honestly... she's a good sized plant, no matter what week she's on, 2 or 3. I think she's just hungry for a little more NPK. Stonington's soil only has an NPK of 0.50 – 0.34 – 0.12 which, for weed, is not much but will keep them going for a couple of weeks - which this plant has now completed. I would start your NPK nutes, whatever you're going to be using, at 1/4 strength and she how she does... and it won't hurt anything to give her a little bit of calmag, too, as you said you were going to try. Seriously, I think she's just becoming a bit hungry! Good luck!