I'd strip the seasoning off of it by washing it with a gentle dish soap and then reseason it.

I frequently strip and reseason my cast iron. I do this by filling the pan with water and about 1/2 c Kosher salt, then set it to boil for around 15 minutes. Then I give it a solid scrubbing while it's still warm. I am perfectly willing to wash my cast iron pans in hot water with a dot of Mrs. Meyers dish soap after each use. As long as you don't soak them, they retain enough season to continue to perform well. People who say they never put water in their cast iron pans don't seem to understand that water is in food, so basically you put water in the pan every time you use it. As long as the pan and water are warm and you rinse and dry quickly, a little cleaning is a good thing.

My regular cleaning for my cast iron involves hot tap water (comes out at 135f) and a bristled brush. If that is insufficient, then the short boil usually coaxes the more difficult bits.
After a good scrub, I always dry the pan on the stove and wipe with a little bit of oil before hanging the pan back up.
My cast iron is usually well behaved when treated in this way.

I'm not a big fan of using soap on my cast iron, but when it comes down to that, I usually go through at least a single stage of re-seasoning.

Strip and re-season. Sounds like part of the seasoning is rancid.

I agree with MonW. Rancid seasoning, fix by stripping (see above) and reseasoning.

Either rancid seasoning or it could be the actual smell of a layer of rust, which can smell pretty strange if you're unfamiliar with it. If you're curious, before using the pan, run a paper towel around the bottom. If you get black, yellow or nearly clear residue, you may have used too much oil seasoning it. If it's orange, that's rust.
Either way, strip the old seasoning and re-season.
http://www.foodnetwork.com/recipes/emeril-lagasse/how-to-season-cast-iron-skillets-recipe/index.html
Some even recommend repeating this process many more times.

I actually store my CIS in the oven on that second, usually unused shelf. Most of the time, I leave it in there when I cook.
1. It seems to even out the heat in the oven when I cook
2. It stays nicely seasoned
3. Saves room in my cabinets

You're welcome, apartment therapy!

I "wash" my cast iron with a salt and lemon juice scrub, rinse with water and then heat to boil off any residual liquid. Never scour them, just use a clean cloth. Then I hit them with a quick wipe of more oil to maintain the seasoning. Never had an issue.

If the pan gives off an odor, it's possible that the original seasoning was poor, there may be rust, or you have a layer of rancid oil (inside or out) which is burning. You may need to take a ball of aluminum foil and use an oven cleaner. Let the pan sit in the cleaner over night then scrub with balls of aluminum foil. You will, of course, need to completely wash and reseason afterwards, but even the worst condition cast ion can be brought back to life this way.

I found to completely strip the cast iron is to place it in the oven on the self clean cycle. It comes out like it was brand new. I do this with my bbq and stove grates. Everything stuck on them turns into easy to wipe ash.

It needs a thorough stripping an seasoning with several coats of fat baked into it in the oven. Crisco works well.

To strip it you can do any of the above or through it into a bonfire and then scrub it. Do not be afraid to give it a good workover. As Alton Brown says: "It's cast iron. It doesn't care." Just don't cool it too fast or it will crack. If you want a cold method, you can submerge it in Coca-Cola overnight, then wash, dry and season.

After most uses, I clean mine with a handful of kosher salt and use a paper towel to scour the salt around until the pan is smooth and shiny.

For the germophobes: remember that the principal sanitizing of a cast iron pan is done by heating it before use, not by scrubbing with soap.

I agree with stripping and re-seasoning. If you're not sure about how to re-season, just cook a few batches of oil or butter sweated onions or veggies in the pan, nothing that will stick or produce a fond. Then just rinse with hot water and dry. A few cycles of that will give you some nice seasoning.

Agree with self-cleaning oven comment. Have done it to address exactly the problem you mention. Works great! Reseason with a dab of Crisco - resists rancidity well. Vintage cast iron is a treasure!

+1 for the bonfire / campfire. That CI will come out looking brand new ready for your own seasoning. When we take our CI car camping, it is so nice to cook on and easy to clean afterwards. Others who camp with us bring an old CI pan to clean and resurrect for their daily use.

The reason seasoning with Crisco (the shortening) works so well is that it's a highly refined fat. Mind you, I'd never cook with it, but I keep a tub in the freezer just to reseason my CIS.

I'd definitely strip that thing and reseason it. Mine never sees soap, but it never smells bad.

The Chemistry of Cast Iron Seasoning

Strip the pan and re-season with flaxseed oil, it will give you the hardest, slickest seasoning possible.

Agreed stripping & reseasoning is the answer. HOWEVER, if you don't know the dif between shortening & oil, do your homework first or you'll be repeating the process soon.