When you paste a headline, it looks like you are really changing two things, bold and font size. If you want to post regular text after that, the system will think you want to continue with that large bold font unless you tell it otherwise. I think the best solution is to paste the headline and then if you're adding text below the headline, type the whole message and then when you are ready to submit, highlight the text you typed and then unbold that selection. After that, the text will still be highlighted so you should change the font size to 4 to make the text normal size.