Shove a clean white rag behind the tank of the toilet using a yardstick or a ruler, then pull the rag out. My guess is that the rag will come out black. The backside of toilet tanks are notorious for growing mildew.
If the rag comes out black, you want to remove the tank from the bowl (a simple task involving 2 or 3 bolts and a disconnection at the shutoff valve) and take the tank outside and wash inside and out with a strong solution of chlorine bleach and water. If that doesn't take care of the smell, you may wish to check the area under the bowl, where it connects to the flange (a marginally more difficult task usually involving two bolts and nuts). I'm willing to bet that will solve the problem.
If you do decide to try this, I'd reccomend that you make sure that the hardware store is open, and that you take the time to replace all your gaskets and rubber washers. They're relativly cheap and easy to replace. Should cost less than $20.00