sewer smell

This winter we have had a septic smell in our basement area-there is a bathroom there. We have lived here 25 yrs and never have had smell before until this past fall. We put a new septic system in in Nov, and are still getting a smell sometimes. We've put water in drains, replaced sink drain, the toilet seal is good. Just recently we have smelled the septic when lifting the lid of the toilet. Any suggestions?