Perhaps I should relate a story here that happened to me a while back so you can check, especially if you recently had work done on your sinks plumbing:
Make sure the trap is still there! My Aunt had a friend do some pipe work for her and he basically made the drain pipes go straight (he took out that little U shape in the pipe). The trap keeps some water in the pipe, which in turn creates a seal between your air, and the pipe/septic. If there is no water there, you could get gas coming back from your sewer/septic.
If you do have a trap, make sure it isn't leaking all the water out too as this could also lead to gases coming back up.
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