since installing a bidet, I have a backsmell in my bathroom when I run wate?

Since installing a bidet a few years ago , i keep getting a backsmell coming from the overflows and plugholes and not sure what has caused it or how to rectify it The soilpipe outside is not as high as the first one but the plumber said it was ok at the height , How can I put this right , it is getting embarassing