Period: Guess who’s back... back again...
Me: Ugh, can we not do this today?
Period: I can come back in 9 months?
Me: Keep fucking singing.
Period: Guess who’s back... back again...
Me: Ugh, can we not do this today?
Period: I can come back in 9 months?
Me: Keep fucking singing.
Hey, dummy! The joke is about menstruation. The "period" is referencing its return and the person's annoyance. The punchline is when the period says it can come back in 9 months and the person replies to keep singing. The person is insinuating that if the period comes back in 9 months, it means that the person would be pregnant. You ain't too bright, are ya?