First timer needs advice

This is my first grow and I have strawberry cough didn’t think they would get this big I see how hard it can be to manage. The girls are coming up on week 4 of flower and a few days straight now I’m seeing yellowing mainly on bottom leaves I don’t wanna lose all this hard work someone help please. This is only letting me put one photo

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Have you measured your runoff and watering mixture PH & EC?