There is a limit on the length, but it's pretty big and I doubt you're hitting it. Break the post into 2 pieces to see if you can do it that way.
You should know if you're hitting the size limit because the message window will stay open, but the program will throw a SQL Server error. If the window simply closes on you without posting your message, you probably included a forbidden spam word.
I can't write any of the forbidden words here because my post will be blocked, too. But just think of the spam you've received over the years, and check your message to make sure it doesn't include any of the typical words, many of which relate to the medical industry. Seeing as how your message is about arthritis, there's a reasonable chance this may be the problem. Check especially for names of medications.
Regards,
Jonathan