That is probably a result of not using "styles", or editing the style to fit your need. I consider "styles" to be the backbone of using Word. Here are some references that will help you:

Understanding styles | ShaunaKelly.com
Understanding Styles in Microsoft Word
Tips for Understanding Styles in Word | ShaunaKelly.com

Of course, there are many other links that will help you but these will get you started.

HTHs...