It sounds like maybe the P-trap on the lav is dirty.
Mix up some baking soda and vinegar until it is a runny paste and pour into drain. Let it sit for at least a few hours, even overnight. This should take care of it.
As a Plumbing professional, please allow me to apologize for the industry as a whole for that smellyness. One day we will eliminate smells from our industry.
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com