What Causes Severe Stomach Cramps Post Menopause?

I am 53 and past menopause. I have not had a period in several years. I have had 7 children, the youngest is 17. Twice in the last several months i have had an episode where I had severe stomach cramps. They feel exactly like labor pains - come on slowly, grow in intensity to horrible pain, and then let off. Over and over and over, almost in perfect timing of about 15 minutes apart. I actually get thru them by breathing like I was taught in lamaze classes many, many years ago. both times the episodes have been at night, starting early in the evening and lasting till early morning about 10 or 12 hours. it is weird and extremely painful