Right-click on the style name in the Styles and Formatting task pane, click Modify Style, and see whether Automatically Update is turned on in the Modify Style dialog box. If it is, Word may have misinterpreted a manual formatting change as an instruction to change the style. Clear the check box.
If this doesn't solve the problem, did you change a style on which other styles are based? If so, those styles change as well, sometimes unexpectedly.
If your styles are still changing improperly, is a template that your document depends on missing? Assume that your document uses a template stored on a network drive. If you open a Word document while the server is temporarily unavailable, Word may use styles with the same name from the Normal template stored on your hard disk.
Perhaps you added manual formatting inadvertently, or imported text that already had manual formatting. Press Shift+F1 and click the text to see whether it has any unexpected manual formatting. If so, select the text and press Ctrl+spacebar to eliminate it (or choose Clear Formatting from the Pick Formatting to Apply scroll box of the Styles and Formatting task pane).
Another possibility is that you have attached a different template to a document, or opened it as part of a master document that contains the same style names but formats those styles differently.