I've been operating under this assumption for years.

In my time as a female I have observed the following related to PMS and "girl time":

-Love of chocolate to the point of craving it.
-Cramps (as if the insides had become angry and were attempting to leave by means of destroying you through implosion).

-Need for comfy pants.

-Excessive squealing over cute things.

-Excessive hunger (example: eat meal, two hours later stomach growling).

-Crying over silly things (Disney commercials, hunger, Muppet movies, David Tutera, etc).

-Watching "chick flicks" (Twilight saga, When Harry Met Sally, etc).

-Desire to talk to and spend time with females in the same condition. aka "girl talk" and shopping.

After nearly a decade of research, I have come to the following conclusions:

-That the uterus is in fact a tiny, fat, stereotypically gay man who periodically gains control but also wishes to kill you and escape.

-If an all-powerful divine being exists and it created us, it does NOT have a uterus.

-I love the Muppets.