A noob appears.

Autos tend to be small before then hit their flower stretch. Pistols are showing with the usual timeline for FB autos, having grown several my guess is you are a week or so away from explosive growth.

It doesn't appear to be anything to be worried about at this point.. just looks like one single leaf is yellow. As far as the red leaf stem - that is indeed indictive of Phosphorus Deficiency but can often be genetic and not something to worry about unless coupled with other signs, which i'm not really seeing. If you're worried about build up or lockout , give her a good flush with plain pH'd water and than I would keep on with the nutes right away. This stage of flower is important to make sure theres plenty of P & K. Good luck