Should I give them a total flush or just run through a couple extra gallons of pHed water

Aiiiiii!!!! Yes, definitely do a hard flush - 3x the water as the pot size (so if you're growing in a 3 gallon, run 9 gallons of pH'd water through). To the last gallon, add your nutes at 1/2 strength (I would do Grow Big at only 1/4 strength) and then let them dry out completely. I would also be checking the pH of the runoff to make sure it's where you want it to be. I use Fox Farm as well and Grow Big is really potent with it's nitrogen content for some reason. I never exceed 1tsp/gallon with it and when my grow gets to pre-flower, I cut that in half and then get rid of it altogether when they go into flower. Since SOME nitrogen is always needed, I add some Tiger Bloom to my Big Bloom - I start out with 1 tsp and then usually increase it to 2tsp but no more... Good luck! I'm so glad you figured out what went wrong! Easy mistake to make, to be honest!