Should I start fertilizing or monitor the pH?

in my oppinion you should always monitor both. But as said by another grower, looks like nutrient burn.

So when switching to flower, it will be looking for more pk. It looks like possible potassium deficiency, however im not sure its deficient. To me it appears this is watering habits. Substrate looks pretty wet and the leaves are folding down with a ripple look to it. Like they're holding too much water. If your mix isnt airy enough, you suffocate the roots and it can cause all kinds of deficiencies or worse root damage. I would allow for some more dry back between watering. Easier to fix an underwater situation than over watered one. To be honest I did not check to see if you have a diary. If you have not fed yet, yes you will need to feed too, unless you're using a prefertilized soil that still has enough nutes in it. Runoff ec test will be good enough to answer if there's anything in the soil. Or do a slurry test. More accurate than a runoff test and keeps what's in the soil in the soil, rather than flushing it out to test. Just my thoughts, Best of Luck! Happy Growing!