You should be able to correct it with a good flush with the proper ph water. You can also continue to water/feed at an adjusted ph until the runoff is correct. Then proceed to use corrected water again.
Make sense?
Here is a sticky which the topic is explained more by someone with much more experience than me.
http://boards.cannabis.com/plant-pro...hy-how-do.html
I asked your soil type because some commercially available soils use certain ingredients that can gradually change the PH over time. Like pine bark chips.