Did you water them the day before they were dense, and then check them the day after? Maybe its just a matter of the Bud being swollen with water, then it dries out? Just a thought...
Could be some sort of light response issue where its like trying to go back into veg? I had a SSH plant last year that started vegging again late into flower - I think its called "bolting". The nice tight dense 2" wide top nugs swelled into 4" fluffy pillow buds in about 3 or 4 days.