So about twenty minutes into a session I wrote my first comment. Published that, and promptly found another interesting pin to add my thoughts to. But I was surprised to be stopped from submitting it. A note popped up saying that I was blocked from commenting because it thought I might be a spammer. This annoyed me because there was no way that first comment was spam. It was thoughtful, comprehensive and relevant.
Anyway, I did some Googling and learned that there are some fairly strict anti-spam measures in place. But they are still pretty buggy, and sometimes identify genuine commenters as spammers.
It seems that if you do a lot of commenting or pinning all at once this raises a flag and it halts your commenting for a 24 hour period, after which everything is reset.
In retrospect I can see why this happened to me. Before writing that first comment I had indulged in a real frenzy of pinning. That must have raised a flag that limited me to just the one comment.
Since then I've been careful to not go overboard with both pinning and commenting. I've just used the site in a leisurely manner and the block hasn't reappeared.
It's worth remembering this when you next log in. Pinterest is not as fast and furious as, say, Twitter. So, work as a slower pace and you will be fine.