I hate to do this... but does it smell like a dead carcass you drive by on the road, or does it smell like a musty moldy odor kind of like a bathroom.
If it has that bathroomy, musty smell, it could be mold. If you have a leak on the outside somewhere and the humidity levels go up, it gives it a reason to grow.
Shine a flashlight down the wall while you hold it against it. You will see all the ripples in the walls from your sheetrockers job, and if you see anything fuzzy, you have a problem with mold.
Otherwise, I'm guessin dead mouse, happens all the time.