PH Problem

HI, I have 3 plants growing in a soil/Vermiculite mix, but have recently noticed that the PH of the medium is around 6.5 when it drys out between watering, but after watering the PH drops to around 3.5, even though I have been carefull in getting the PH of the water/Nutrient exactly right, the run off water after soiking through the soil is about right 6 -6.5. The plants dont seem to be suffering from any problems, however I have noticed that a few of the top leaves have developed what I can only explain as small dimmples on the ends of the leaves. can anyone help explain this. I am a fiest time grower, and the seeds came from a stash of Bud, I am not sure what strain but decided to grow them as an experiment before moving on to some serious growing. they are currently about 7-8 inches high with lovely bushy foliage.

Thanks in advance