Indeed it can be natural during the flowering state. The plant is using all its store food from the leaves to produce the buds. So it starts taking the nitrogen, first from the bottom leaves to feed itself. Pictures would definately be helpful. The pH strips are a pain to use. My advice, if you dont have a digital pH tester is goto a petshop and buy a water pH tester for fish aquariums. They give you a small vial and drops to put in the water. That will give me a bit more accurate reading. If your pH is 7, it is definately on the higher end and can cause nute lock. The preferred pH range for marijuana is 6.3 to 6.8 with 6.5 being ideal.

GTD