my girl friends breast are getting larger after her period . this has happenned with her last three cylces ? i

According to MedlinePlus, breast tenderness and swelling are normal parts of menstrual cycle:

"Breast tissue may have dense, rough, "cobblestone" feel to the fingers -- usually more marked in the outer areas. There may also be an intermittent or persistent sense of breast fullness with dull, heavy pain, and tenderness.

During the menstrual cycle, estrogen production increases and peaks just prior to midcycle. This causes enlargement of the breast ducts."

Is your girlfriend concerned about this, or were you just curious why this occurs?

If your girlfriend or you are concerned, there are a few warning signs that would indicate that she may want to see her doctor:
* new, unusual, or changing lumps in breast tissue.
* one-sided (unilateral) lumps in breast tissue.
* woman, age 40 or older, and have never had a screening mammogram.
* discharge from your nipple, especially if it is a bloody or brown discharge.
* symptoms are interfering with your ability to sleep and diet changes and exercise have not helped.

You and your girlfriend can read more here about recommendations on how to lessen breast tenderness and swelling.