Hey susan,
During her period, my girlfriend gets major pains in her stomach and turns purple. Why is that and what can she do to stop it?

Not a whole lot she can do to stop the pain.  Some medications like Midol work. It help not to eat the salty food while you are having your menstrual period. Warm baths help for a little while.  If she is having orgasms - that helps more than anything else.

What part of her turns purple?

Susan