You gotta give more information.
Outdoor temperatures and barometric pressure both influence how well a chimney draws.
How well seasoned is your wood and are you sure it is kept in a dry area?
When was the last time your chimney was inspected?

This happened to us too - it might be as simple as the problem we had - somehow our grate got pulled too far forward so the wood was angled too far out causing the room to smoke up. Be sure you grate is all the way back, pushed up against the back wall and keep all your logs towards the back of the fireplace. If they fall forward, the smoke will start to permeate. Hope that helps.