One thing that can happen in homes that have and use fireplaces is that when the house furnace kicks on, either because other heating zones in the house create the demand, or for water heating, the combustion air can be pulled… from the fireplace flue.
Whenever the furnace starts, it pulls in combustion air from inside the dwelling, uses it to burn the fuel in the furnace (whether oil or gas), and sends it up the furnace flue. It used to be that houses weren’t so well-sealed, and outside air would find its way through small window casing openings, under door thresholds, and through imperfect seals in the walls themselves. No more. Now that houses are wrapped in Tyvek and window and door seals are so good, there’s much less air infiltration. But the combustion air is still being used in the furnace and is still going up the flue. So what is happening is that the home has a negative air pressure, and it sucks air from from outside wherever it can. If your chimney flue is open, then it’s the most likely place.
So your furnace is probably pulling some of the smoke down the flue and into the room to equalize the air pressure in the home.
There are a couple of ways to avoid this.
1. Be sure that the furnace won’t turn on while the fireplace is being used by simply turning it off until hours after the fire is out.
2. Isolate the room with the fireplace from the rest of the house (may be difficult in a living room, since most don’t have doors that you can close and seal), or
3. Provide a direct air inlet to the furnace from outside so that it’s not pulling ‘room air’ in. The house will maintain its neutral air pressure and not try to suck any more in from outside to make up for the combustion loss. (This cold air inlet to the furnace will reduce its efficiency somewhat, since you’ll be mixing frigid air with the fuel instead of pre-heated room temperature air, but it’s far safer for you than the situation you have described, which can kill you – as well as potentially suck hot embers down the flue, out of the fireplace and into the room, which is also no good.)
4. It may be possible to install a damper in the fireplace flues that prevents the reverse flow of smoke and air back into the house via that route – and maybe you already have that and my advice above is all wrong – but I’ve lived in houses that don’t have such a device, and I’ve seen it happen.
PS: Welcome to Fluther.