Now that I think about it... I think I know what you are talking about...
I have had it happen a couple times, but it has been a while. It was with some c99 plants.
It was right around 1 week before harvest. The calyx were swelling like normal, everything was good, then all of the sudden the buds shrunk. It sort of looked like they lost hydration but the soil was wet.
I never did find out what the cause was... It was shortly before I got root aphids for the first time. But I don't know if they are connected. Possibly I had the aphids at the time but did not notice until the next round.
My guess at the time was that the sativa genetics in the c99 were showing a closed window of harvest. Like I waited just a little too long or for some reason the window closed. I never waited long enough to see if it would open up again and the calyx would swell. so...?
I would take a look at the root system. See how your roots look and check for pests. Your plants can become dehydrated due to root damage, even though your medium is wet. But then I would think that your leaves would be showing signs too.
Without pics, you're not going to get any good answers. Just post when you can get to a computer.
Post in the plant problem section, with pics. You'll get more info there.