I have an 8 year old shih tzu who was never formally trained but was trained to ...
Congratulations on the upcoming family member!
I'm a little unclear on the timeline of events... From what I gather, before you were pregnant, you would catch her pooping while lying down? I'm assuming this was indoors, not outside. Then 3 months ago, when you were three months pregnant, she began pooping inside all the time rather than just occasionally? She pees outside now, and always has, but seems "scared" to do anything else... how can you tell she's scared? Are there visible signs - shaking, panting, drooling? Does that mean she acts scared outside after she pees and that's why nothing else happens? Does she spend other time outside, playing or lounging around?
The situation sounds unique, so I have lots of questions. If I'm guessing right, it sounds like she was never completely housebroken (i.e. no accidents in the house), but it has become worse and she has become nervous outside. Additionally, she has no problem eliminating in her kennel. If this is correct, I would make sure the kennel is small enough to only stand up, turn around, and lie down. I'd also make sure there's nothing absorbent in there. If the kennel already matches this description, then you'll need to go with a bigger area and probably some sort of indoor potty option (like potty pads) for a while. You'll want to treat her like a puppy as far as supervising constantly and taking her out to potty, plus rewarding with a treat when she does. If she has an accident, don't scold her or that may contribute to her anxiety. If she's nervous being outside, try to determine why and then make outside more fun. This might mean you need to spend more time playing in the backyard, feeding out there, training, etc., so she learns that nothing bad will happen there. Feel free to reply with more details and I might be able to give more specific suggestions.