You could have many possible causes. One item you can easily check is to make sure your fuel cap is secure. One area you usually notice no symptoms with is the evaporative emissions system, which requires a secure fuel cap to seal the fuel system. Other issues can be present in that system, but the fuel cap is the best place to start looking.
Other than that, you simply need to have a qualified technician help diagnose it for you. Here is a link of local shops near you. Good luck!