I have an idea- a person is likely to eliminate accumulations in the large intestine (colon) and possibly even some in the small due to the lubricating and balancing effects of castor oil. That along (uneliminated old fecal matter) will cause a pooch.
You mentioned your wife used the pack on her "abdomen", however menstrual cycle discomfort may be addressed by lowering the pack to the pelvis, over the uterus and ovaries, below the navel and in the vicinity of the pubic bone. She may have had it too high, although of course there are other contributing factors to cycle pain.