Nitrogen deficiency

Depending on where abouts on the plant that leaf is ? If its at the bottom that wouls make me think nitrogen if its at the top I would say more of a ph Fluciation though by looking at the start of the bud development at the top that would lead me towards nitrogen aswell. So yeah I think your right check her ph and if thats okay try increasing her veg feed a lil more our ladies dont use much nitrogen in flower this is true but N like P K and cal mag are mobile nutrients within the plant meaning they are in constant use by the plant regardless of which stage of growth she is in :) Hope this helps look nice and healthy anyway. Phosphorus defienacny will usually turn your plant purple I know this because when you flush a plant the purple you see is actully the plant consuming phosphorus that's in the leaves turning them purple. Purple or red stems is the plants natural reaction to intese light so I would not worry about that. Though this is all my opinion and only based on my experience you may disagree which is fine :) Best of luck anyway im sure you will finish these ladies up with no more issues looking good :) 👊