Hi there,
Is it possible that you composed the entry in another word processor (like Pages), and then copied and pasted it into the text editor? That can occasionally throw up some weird code issues. Other things that can cause strange quirks are using another app (if you're on an iPad or other device).
If any of that is true, that could be the culprit, even if it's worked fine before! If you've just been composing in the Edit Post page itself, that's a little bit more murky.