Our house is 1 year old and there is a sewer/rotten egg smell coming from under our sink. We have a dishwasher & garbage disposal. The smell seems the worst after I run the dishwasher and then use the sink. What could it be? Please help.

Usually, this is a dirty disposal. Try cleaning the disposal with one of the following methods.
1) Salt - dump half a box of salt in a running disposal with no water, then turn on the water
2) Ice - With a disposal running, with the water running, dump an entire try of ice in, 4 or 5 pieces at a time, letting it clear before dumping more.
3) Soap - this is the riskiest - fill the sink with warm water and put a good size squirt of dishwashing liquid in it. Start thie disposal, then pull the sink stopper, let the water swirl and completely go down the drain, then turn on more water.

One of these should work.

I have used these methods to clean my disposal, but the smell still seems to be there. At first I thought it was just a dirty disposal, but now I don't think that is the cause. THank you.