It’s probably just caused by hot weather.
In the summer when you are not using your fire, the air from inside your house gently rises up and out the top of your chimney. When it gets very hot, the difference in the temperature between inside and out can cause the chimney draw to reverse, bringing the air down the chimney and into your room. Even if your chimney is clean, this will bring a strong, sooty / tarry smell – like your clothes if you’ve been near a bonfire.
What can you do?
If you are not too bothered by the smell, do nothing and it will disappear later in the day as it cools down outside. If the smell really bothers you, try lighting a few candles or tealights in the fireplace/ stove. The gasses from burning candles may well have enough heat energy to overcome the downward air and move everything up and out the top again. Ta-da. No more smell.
But if it’s hot outside, you are unlikely to be using your fire so it’s a great time to call your local professional chimney sweep. As well as cleaning your chimney, they will make a number of checks and offer useful advice which will help save you money, keep you safer and help with efficient burning.
There are occasionally other factors that can cause a sooty smell so if it persists beyond a hot weather period or it’s happening in winter, it’s best to ‘Speak to your Sweep’.