If it's a swamp there might be gases being released: https://www.sciencealert.com/how-this-small-lake-in-africa-once-killed-1-700-people-overnight-and-we-still-

If it's a swamp there might be gases being released: https://www.sciencealert.com/how-this-small-lake-in-africa-once-killed-1-700-people-overnight-and-we-still-don-t-know-why https://www.sciencealert.com/how-this-small-lake-in-africa-o... But yeah the swamp might not have anything to do with it and it could also be something in the house, or the drinking water, etc.