I was working in a writer document (consisting entirely of text), put my computer to sleep when I took a break, and on returning to the document found that anything I now added was underlined, the color was yellow instead of (default) black, and marked “Inserted Author Unknown” even though I am the only author (this is my PC at home and no one was present to change any setting). I am using LO 6.0.7.3. Thanks.
It seems you accidentally activated the track changes feature in Writer. Go to Edit → Track changes to stop “recording” and to accept the changes already introduced.
The same happened to me and the reason was I pressed Ctrl-Shift-C, trying to do a “copy format”, as is done in MS-Word. LibreOffice does well to remind us that it is NOT MS-Word. 