When you think you figured out the PH, then problems appear..

The PH pen calibrated- do it once a week.

First thing is to recalibrate your pH meter and remeasure. You should also be measuring your EC. Most cases, your best course of action is keep feeding between 6.5-6.8 with lightly fertilized water. I'm guessing your medium is too hot, but you dont want to risk starving her either. I would feed at about 400-500ppms until everything stabilizes. When the EC of the runoff hits the level you want (probably 700-900ppms), you can raise the feed to that level. If the runoff ppms start going up, lower your nutes a little. If the runoff ppms start going down, raise your nute levels. Pay close attention to this and you'll always be spot on for feed strength if we assume you're using the correct ratios.