Is there anything wrong?

Hi
I have 3 hybrid hens who I bought as pol about 7 weeks ago. They started laying within a week and gave me 3 eggs a day until about 2 weeks ago. They have all stopped laying. We have had about 3-4 very small eggs in the last fortnight. They seem healthy to my inexperienced eye- their behaviour/habits haven't changed. At first I put it down to a cold spell, but now it's glorious.
I have checked them for lice etc but they seem fine.
Any ideas? How long should I leave it before taking some course of action?
thanks