Writer. When I change the title to italics (or bold), all document changes to italics (or bold). How do I change this?

LibreOffice Writer. Sometimes something changes in my document so that whenever I change style of one line of the document, the entire paragraph or indeed entire document changes style. I can not possibly resolve this problem on my own. I had the same problem with MS Word, and the solution for that was really complicated. Please help me!

I am using LibreOffice on Linux (Ubuntu LTS 14).