its ok smoke the plant is only using up its own Nitro , all plants yellow during flowering , its what u want then to do . one thing i would say is to add more perlite to ur soil mix , marijuana needs good drainage , try a 50/50 mix the next time . getting back to the yellow leaves , ur plant doesn't need as much N during flowering as it uses this to make leaves and stem ( not flowers ) it still needs sum all the same and if it can't get it from the soil its will take it from itself . in the last 2 weeks of flowering i will be using GH ripen and its has next to no N at all . u want all or fans leaves to yellow before u harvest.
ps the bottome leaves are yellowing 1st cuz the top of ur plant needs the N
I hope this helps
SH