the lower leaves yellowing could be a couple things but most likely lack of light if you are talking the "undergrowth". Nothing grows in the shade so if you have a big full dense canopy the anything uner it is gonna yellow. Eventually leaves under that canopy that are not getting sufficient light will yellow, die and fall off so I thought i would warn you :thumbsup:

your PH change could also be a real simple thing. You just went to flower so you are readjust the plants biological clock and during this time the plant is like wtf?? :wtf: I use to get more light where did it go and I use to get grow nutes, where did that go??? When just starting flower or transplant or anything that cause a change (stress) to the plant, it will be more apt to go through more water than nutes and a rise in the amount of nute strength will no doubt change your PH. All will be good, you have been doing real well at staying on top of things and I have no doubt you will enjoy the fruits of your labor :jointsmile: