15 C at night is a bit cold. Are these the same temps as before you started flower? You see what I'm saying; if the temps were this cold in veg, that's one thing. If for some reason it's colder now, that's something else entirely. If you went to 12/12 from 18/6, the plants are spending longer time without light so I guess that would mean they are exposed to more cold.
Can you stick a little heater in there? On nights where the wood stove wasn't quite keeping up I've had good results adding an electric space heater. The plants seemed happier in the morning.
If it is temperature related, and you add heat (shoot for roughly 25 degrees C or more at first) you'll know within 24 hours. When the plants are cold, all of their functions slow down. Transpiration, respiration, etc.