Heat really isn't the story here. Hens eat more to stay warm in winter, especially if there's nothing wonderful out there in the snow. BUT it's hours of light per day that actually impacts egg production most. Sixteen hours of light (can be dim) per day is what maximizes winter egg...
Your hens may be starting to molt, and the days are shorter now. Do you have extra lighting in their coop, 4am to 8am? Hens want at least 14 hours of light, better 16 hours daily, to lay eggs. Many hens will take the winter off without those extra daylight hours.
Mary