She could be!
Mine spent a coupe of weeks 'playing house' in the nest boxes before they laid their first eggs.
Does she show any other signs of impending laying?
Is her face bright red?
Are her combs and wattles bigger than usual, bright red and sort of plump looking?
Will she squat when you come near her?
Does she seem more vocal than usual? Mine were sort of anxious and squawking near the nest boxes for a couple of days before laying.
They were also in/out/in/out of the coop all day long! Sometimes they even bought our cockerel inside to investigate the nest boxes with them!
Does your cockerel mount her (if you have one, that is?) Mating in our yard started when the girls were coming into lay too.
These are all signs. You might see some of them, or all of them. At 18 weeks old she will certainly be preparing for it though.
Have fun looking for eggs - it won't be long now! Krista