You may wish to check to see if the paragraph itself has italics enabled before pasting. Even though you may have deselected/turned off italics, Word (most versions) has the tendency of restoring the previous formatting to any text that is pasted there if no other text has been typed in in the interim.
A way to tell is to show paragraph marks (
Ctrl + Shift + 8). From here, you will see that any new lines will have a paragraph symbol (it looks like a mirrored-'P' with two vertical lines supporting its spine).
Another possible way of avoiding this is, once you have a new line with NO formatting, simply enter a single space, and then paste your text in. This will cause Word to think that you have typed some content (which you have) and that you wish to proceed with the current formatting.
Lastly, you may wish to try 'Paste Special', which can be accessed via the shortcut menu on the 'Paste' button, or by pressing
Alt GR + V. From here, select
Unformatted Text and your text will be pasted in the most basic format. Even though you say that you have copied the text from Notepad - which I agree is the best way to ensure that text has no formatting - Word can still be a little unpredictable in its behaviour and the way it renders text from another application.
I hope one of these methods helps.