Patients who breathe through their mouth tend to have a characteristic posture, hunched with the head tilted forward. This helps them take in air more easily. In this posture, the chin stays pressed down and its forward growth is restricted. That is why mouth breathing is also characterised by a lower jaw that lags behind in its growth.
A correlation has been found between a crossbite on the side teeth and scoliosis (an S-shaped curve of the spine). If both conditions are found at the same time, the recommendation is to start widening the jaws straight away.
This answer is for general information. Your own case needs an examination by an orthodontist.