Hi papajoe, welcome to the forum! 
There is no need for panic. 
During a fishless cycling, it is perfectly normal to have the water PH suddenly crash when you are in the stage of high nitrate production.
The production of nitrate is acidic, and thus it slowly eats away the water buffer. Once the water buffer is gone, the PH will suddenly drop. I have seen PH of 4~5 overnight from PH 7~8. It is actually supposed to happen sooner or later. If you are in an area with hard water, it would happen later. Obviously, it would happen earlier if your water is soft with little buffer.
There is nothing to worry about. All you need to do is to do a large percentage water change of 50~70% to remove the nitrate while restore the water buffer. The PH should go back to normal range.
It has to be noted that the water change should come as soon as possible after you have noticed the PH crash. The good bacteria you are trying so hard to seed the aquarium does not do well if the PH is way too low, and the fishless cycling might stall if the PH is not restored.
In our article of fishless cycling, we have gone through exactly these problems. 
It is nice to see another fellow fish keeper doing fishless cycling before getting the fish. If you have any further questions, feel free to ask. 
Good luck!
| 
