When your drains are getting smelly it's usually due to a backup somewhere in your drain pipe system that is causing sewage to not flow as freely as it should and then this sewage backup causes sewer gas to back up into your sinks drains.
Your P Trap is supposed to prevent the sewer gas from coming into your home but sometimes the water evaporates out of the P Trap.
You can try pouring some warm vinegar down the drain to help get rid of the smelly drain and also pour some boiling hot water into the drain after you let the vinegar sit in the drain for around 10 minutes.