Why has my text gone all bold / italic ?

This is probably because you have used the ‘Read more…’ link.

Check this image:

Notice that the em tag opens before the --more-- tag and closes after it.
That is making the error happen.

Now check this:

The em tag is now closed before the --more-- tag and so your text will be normal.

Click the Code tab to edit your post when this happens so you can see the tags.