Hey mate,
yes your guessing right. Thats a slight nutrient lockout due to a wrong pH. Your plant cant take up the nutrient it wants, because your pH is out of range.
You can use citric acid to lower it, when i dont have ph-regulator i simply use lemonjuice. a teaspoon lowers the ph for around 0.5 on 1Liter.
Happy Growing, dont stress it👍