It could partly be the stress from the fowl pox and partly from the reduced daylight hours and cold. Winter is normally their break time and depending on the breed there are few to no eggs laid. The most common eggs in the winter time are from production breeds and people who add extra light for their birds. If she is not acting funny and doesn't feel egg bound then there is nothing to worry about. I highly doubt egg bound because she would be much worse acting right now but if you want to check you just feel under the vent and see if you feel an egg inside her. As the days get longer she will start to lay again and may have a few small eggs like when she started laying, then she will be back to normal.