Not sure if you're using Tiger Bloom, but it seemed to be the culprit of my nute burn during my own flowering. Gotta go really really light on that stuff, especially if your soil already has fert in it. I'd say the Big Bloom is good to go up to 75% of recommended dose. FF even suggests using Big Bloom along with their Sledge Hammer product when you flush because Big Bloom cleans your roots. It's essentially a minute amount of NPK and worm castings. Some other shit too. Anyway, if you're scared of nute burn and you're using Tiger Bloom, that's probably what's doing it.