Next possibility then is overfert. How much guano did you add? Overfert is funny sometimes- your plants might tolerate a higher fert level right up to the day when your temps hit 95'F and then BAM you see nasty burns. The increased heat will definitely make fert burns show up faster or more severely.

So since you have just flushed, give a nice LIGHT feeding of your base fert, control your temps, and start counting down to harvest.