Well there certainly is good reason to be concerned.. That sulfur smell most likely has Carbon Monoxide with it which can be deadly if there is enough exposure. I hope you have 2 or 3 carbon monoxide detectors in the household. I suggest hunting for the problem and solving it without fail, immediately. It should be shut down until you can work on fixing the problem. Firstly, the chimney should be checked for a blockage or breeches and a manometer needs installed permanently to make sure there is always negative pressure in the chimney/flue pipe.
Don't take unnecessary risks with carbon monoxide.
If you don't have any detectors, get at least two of them ASAP.
Please keep us informed of your progress so we can help fix the problem...