Plumbing Questions? Ask a Plumber and Get Answers ASAP.
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I suspect you have a broken vent pipe or possibly a joint has pulled apart based on your description. I sorry to be the bearer of bad new but you'll probably have to open up the wall to find it. I'm assuming that the plumbing was done at least somewhat correctly and the fixtures have traps. It's also possible that your washing machine drain is not vented properly and because of that the trap is siphoning out when the washer is draining. This will also sewer odor into the house. One way to avoid a lot of extra work would be to hire a plumber who has a boroscope (see attached pic). With a boroscope a smal hole can be drilled in the wall to insert the probe to look inside the wall without tearing it apart.
I'm sorry to hear about your difficulties with an unscrupulous contractor. I'd suggest you have a plumber come out and check out your situation. Generally a broken drain pipe won't cause an odor problem because there's usually no way for the odor to escape through the slab unless there are holes in it. The slab shifting could break a drain line. Also your description indicates to me you probably have several issues with the plumbing not installed properly. Intermittent sewer gas infiltrating into the house isn't likely to make you sick but it needs to be addressed.