Y'know, something like this happened to me when I was trying to post an idle thought thingy. The spell checking feature caused me to lose it twice, so I finally threw up my hands in despair and just posted it without using the spell checking. I am quite positive that I had entered a subject line too.
?!?
Well, I finally heard back from Spellchecker.com Support, but not surprisingly they say they have no idea why the text of random posts might disappear after they've been spell checked. :-/
One thing I've noticed, though - and it doesn't necessarily just happen after a spell check - is that the posting window will occasionally appear to be blank when in fact it isn't. If you hit the enter key 1 or 2 times, or you click preview/refresh preview, what you've previously typed comes right back. So, the next time it looks like you might have lost a post, try that out before you panic.
And sometimes you might also be able to navigate your browser back to a previous version of what you've typed.
I'm afraid that's about all I can suggest because, as I said at the outset of having installed the spell checker, the program is handled off site by Spellchecker.com, so I can't even try to figure out what might be causing it to act weird every once and awhile.