My first cousin had similar problems. I don't want to scare you but she has four children... the first was a single birth, a few years later she carried identical twins (girls), and finally she had a boy before the pain she felt during her cycle and then it became all the time turned out to be the result of a distended uterus. Her uterus, as your Nanny sort of explains, had become enlongated and was coming free from parts of the inside of her body. The surgeon attempted a tacking procedure but toward the end my cousin's uterus was so badly damaged, particularly due to the complications and the weight of the twins, which she carried to term oddly enough and they were of great weight unusual for twins not to mention getting pregnant again a few years after having the twins. It was nothing more than the luck of the draw. Her body just couldn't handle more.
It's not so unusual in our family but then again, most often many of the women on my mother's side are fortunate if we can get pregnant, stay pregnant, and then carry to term. I was never supposed to be able to get pregnant (slim to nil chance....). Between the course of 10 years, I was able to get pregnant and even though these two pregnancies were severely high risk, required bed rest from the 3rd month, and I spent more time in the hospital than at home, I was able to have children and carry them until they could be born safely and oddly enough, healthy for the most part although my oldest was born with strabismus, which is crossed eyes that had to be surgically corrected by 7mos but it worked and then a few years later she began to have heart problems, which later turned out to be paroxysmal atrial tachycardia with an irregular heart beat, which medication takes care of as long as it doesn't become worse. My youngest so far is healthy.
I'd find a second-opinion OB/GYN and/or possibly a surgeon to go in for an ultrasound, CAT scan, or maybe even an MRI to find out if there is something simliar to what my cousin went through going on or even a check for ovarian cysts, uterine cysts, or even endometriosis just to be sure. There are so many things it could be and it couldn't that it's best to seek the best professional possible to find out.
Don't let the pain become too bad before going to the doctor. It could be something very simple or something very nefarious so check it out either way. You're too important to lose over something that could be easily fixed or repaired, whichever the case may be.
Take care and I'll keep you in my thoughts.