You're impossibly fast. And strong. Your skin is… pale white, and ice cold. Your eyes change color… and sometimes you speak like - like you're from a different time. You never eat or drink anything; you don't go into the sunlight. How old are you? Seventeen. How long have you been seventeen? …a while. I know what you are. Say it… out loud. Say it. Vampire.

This means that her cycle is 28 days exactly (16+17-5) and that her last period began on July 28th 2006. That was also her last period EVER btw because then she got pregnant and then was immediately changed.

So I counted backwards to every single one of Bella’s periods, from her last one before she changed, to the first one after she moved to Forks. The results:

To elaborate, Bella was on her period:

When she first met Edward at biology class (January 18th 2005)

At the meadow (March 12th) and baseball game (March 13th)

The first time she heard Edward’s voice in NM (January 16th 2006)

When she bought the bikes and she started repairing them with Jake (January 17-18th)

When she and Jacob did homework together and she invited him to the movies (February 14th)

The day she realized Jacob is a werewolf (March 11th)

Quite possibly when she jumped off the cliff, depending on how long it lasts for her (March 16th)

When  Jacob confronted Edward and Bella about the treaty and gave Bella back her motorcycle (April 7th)

When Edward caught Riley’s scent at Bella’s house (June 2nd)

At the Quileute bonfire (June 3rd)

When Jacob went missing in BD (Late June/Early July)

“Bella!” Edward called out in acute relief. We’d caught him in the middle of pacing back and forth across the little open space. He flashed to my side, sort of blurring as he moved so swiftly … and caught me in a tight hug.