After heavy use of your garbage disposal, the blades inside the unit often times get dull. In addition, unpleasant odor may also come from your garbage disposal for not cleaning it out periodically.
Taken a few cups of ice and either rock salt or vinegar and pour as much as you can into the garbage disposal. Then run the disposal where the ice will help sharpen the blades in the disposal and the rock salt or vinegar also becomes an abrasive cleaning the inside of the unit.
After you are done with this step, cut up a lemon peel and pour it into the disposal along with some warm water. Run the disposal which will help eliminate any odors. After this, fill up the sink with a few inches of water and then let it flush the disposal and the sink trap drain below.
Here is a video from Howcast on these steps.