Hi, I'm Dr. Deb. I'll do my best to help you today.
I'm sorry for this concern for Ellie.
This is going to be a hard question to answer unless you could find something rotten or partially decomposed in your yard...but this should be quite obvious if something like this is present. Dogs will also roll in and eat mulch/compost which can be toxic
to them if it's fermented.
Mushrooms are also something to consider although they don't usually have a strong smell.
I do have a suggestion for helping to remove the smell. This is what I suggest for dogs who have been skunked and it works quite well at removing offensive odors:
Tsp of liquid dishwashing liquid (Dawn is the best choice)
1/4 cup of baking soda
One quart of 3% hydrogen peroxide
(Dispose of any left overs..the mixture might explode otherwise)
First, bathe Ellie in Dawn dishwashing soap. Rinse well.
Then use the peroxide mix. Rinse well.
Bathe her again with the Dawn.
Rinse really, really well.
You do want to avoid getting any of the mixture into the eyes (as you would anything not intended for use in the eyes) but it's ok to use around the nose and mouth.
Another option would be Massengill vinegar and water douche--the kind you mix up from a powder. Wash her first in Dawn dish soap--rinse well--apply the douche and let air dry--do not rinse off. You can use a spray bottle and cover the eyes with your hands or a washcloth over the eyes.
Hopefully, whatever she got into/ate is gone now but a close examination of where she tends to spend her time when outside would be prudent.
I hope this helps. Deb