Perhaps it's the seasonal change. By that I mean our temps shot-up after last week's storms. Very warm including the water. Warm water is a gold mine for algae. So we have to do two things as we approach summer:
1 - Ensure we have adequate CYA protection from the sun
2 - Continue to increase FC for that particular CYA
I understand about never dropping below an FC of 4, but if your CYA can't protect the chlorine, it may have dropped quicker at some point (maybe). If by chance your CYA is actually a bit higher than you thought from all the work before, you may need a higher FC range. You did the right thing by increasing immediately. Simply confirm the CYA first. If it's still between 35-40, then the FC of 16 is fine. Once you complete this round, you might consider increasing CYA to 50. Of course you would have to increae your FC range as well on a normal day, but your pool may demand it because of the increased UV intensity.