Good day everyone

I have a bit of an odd problem. Whenever I apply formatting to a line of text, such as making it italic or increasing its font size, it applies it to the entire document. And it is on every new document that I make. The weirdest thing comes after. I press control z to reverse the effects, and everything goes back to normal, except the line of text I wanted to format. That stays, for example, bold.
If anyone knows what is going on, please tell me. It can be annoying at times