test post error when previewing new page–messed up whole site

Hello–
When I try to add a page to my site, including adding an image (content image) retrieved from my media library, it doesn’t work. Instead, the page loads with no image and with the following text: “This is a second post. This is a test post.” And this happens EVEN IF I HAVEN’T PUBLISHED THE PAGE. In other words, if I just set up a new page and preview it, I get this error.

Worse, this one action seems to mess up all my pages, except my home page. Every page loads the same way–no image and with the “test” message (exactly as above). Again, this happens even if I don’t actually publish the culprit page.

To resolve this, I have gone to Settings–>Permalinks and then switched to the “default” setting, and then back to the original setting (name and month), which seems to reset things.

Would love to know why this is happening and what I can do to resolve it! My site is one I am constantly updating and I feel paralyzed to do anything to it now.

Thanks for any help.