It's most likely heat damage because of the irregular brown spotting that shows on the leaf. It's either that or bugs are sucking the juices from the bottom side of the leaf. That can also appear as random brown spotting, as the bugs attack without any direction to them and can kill small areas of leaves. But the heat damage idea is good too as that is exactly what heat damage looks like.
pH actually tends to be the least of a grower's worries, provided the soil and nutes are good. The only symptom I've ever seen in a plant with pH problems is they simply stop growing. But then I've never seen an advanced case of imbalanced pH either.