The slight swelling and sensitivity of the
breasts which a
woman experiences for a few days, just before having her menstruation.
The word "periodical" refers to both the cause of the increase in size and the short duration.
As opposed to "major boobage", the effect is
minor and temporary.
Man:
Wow! Your breasts have really gotten bigger! Did you
get implants?
Woman: No, they are only periodical boobs. They'll be back to normal in a few days.