Okay...I most definitely could be wrong, but... I think it has a lot to do with just trying to get the machinery working again on egg production. My girls are new layers, and today my BA was acting that way (three of them spend quite a bit of time in the nest box, whether an egg is produced or not). She was acting as if she was trying to get something out...tail and behind, around vent area, working steadily...as you described it pulsating. I figured she was going to give me a shelless or soft shelled egg (three of mine have had malfunctions since they've started laying in the last two weeks or so). I finally left her alone and came back out an hour later. She was out of the nest and walking around with the others. Like you, I felt no bulge...nothing back there. I think their systems are trying to get cranking (in your case again)... I hope I'm right.