Yea it can be any number of things without knowing the pH and ec/ppm... You should def get both, they're essential. But you might not even have nute burn because some deficiencies will actually show burnt tips, just like nute burn. You're pretty late into flower at this point so instead of chasing your tail guessing what it can be I'd just make sure environment is right, add more calmag, and just keep your nute solution at the recommended dose. Def order those meters. $15 for an adequate ec/ppm meter and I wouldn't spend less than $40 for the pH. Gl