Hi Donya,
are you maybe using soft line breaks (Shift+Enter) for new lines? These are designed to keep the current style and count following lines towards the same paragraph. When you change a paragraph style, everything in that paragraph will be affected by design. That’s what paragraph styles are good for 
Let me show you what I mean:
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I start writing my chapter heading
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I hit Enter and continue writing. The style changed automatically.
Now when you press Shift+Enter instead, you get a soft return, which keeps the current style until you insert a regular return.
I insterted soft line breaks with Shift+Enter and keep the style for all these lines.
You can see what kind of line breaks you are using when you zoom in a little:
If you don’t see these, make sure you have them enabled in the View Menu.
You can also insert different types of line breaks (like that soft line break) from the Insert Menu.
If that doesn’t help, you can send me your document in a private message and we’ll have a look.