Why has some of my text changed to bold?

For some reason some of my text has changed to bold – it seems to have affected all body text and the sidebar. The only thing that isn’t affected is whichever post is most recently published.

I just installed 3.5.1 but this problem began before that. Anyone any idea what’s happened and how I can fix it?

My blog is: https://www.lifestyleflash.com/

Thanks.