Adjusting position of sentence

Hi 

I have used 'Word' to write some history. All of a sudden when I highlight a sentence that I want to have in 'bold', the whole document is put in 'bold'. I then have use 'undo' to get my document as it was, then I find that the document is as I wanted, and the sentence is in 'bold'.

The same happens whe I want to centralise a sentence. I highlight the words, and all the words in the document is centalised.

I 'undo' and the document is back as it should be and the words are centralised. 

So annoying and time consuming.  What has happened?

Thanks in advance.