The jaw pain during pregnancy can be because of the following reasons.
If any one of you are sitting with your chin propped on any of your hand for a longer period of time then it can lead to jaw pain.
If you have slept on your stomach. I know it is not possible for you to do that by accidentally if you would have slept then.
You should refrain from grinding your teeth on one another. This can also lead to jaw pains.
The other reason can be if you try to bite something which is really hard or got hurt accidentally then it can also lead to pain in your jaws.