Deficiency or excess?

Yup. I had the very thing happen just like that on my super lemon haze on week 3 or 4 I think. I let mine get much worse you are catching it earlier. So you are running soil right. So a calmag agent will be a need to get especially with an auto. Or if you can get your hands on any epsom salt and do a low 250+water ppm and that will be a good source of mag sulphate. And for cal if you try dial in your PH and anything you have with a bit of cal in it just give. But a calmag of course would be easier to get. She's an auto so you are going to need calmag the more you get into it. Also I'd say you would want to cut the grow nutrients now and start pushing bloom. Happy growing.