It’s exactly what you stated above. Period cramps stem from the uterus as it contracts to get rid of the build-up that the body gathers to prepare for pregnancy. If no pregnancy, the uterus has to contract in order to remove this “endometrium” from the body. As you stated, period cramps can radiate all the way to your knees. The vagina is in the “in-between” area and sometimes the cramps will radiate through the vagina. The vagina is very strong but also very thin, and full of nerves. You may just not have noticed them before, or the uterus may be working harder to rid itself of the endometrium; making the uterus contract more heavily and your cramps worsen.
Try an anti-inflammatory such as Ibuprophen or Naproxen Sodium. These can be found by their generic names or brand names (Ibuprophen – Advil or Motrin, Naproxen Sodium – Aleve). These anti-inflammatory pain relievers can be bought over-the-counter, but if the pain is very bad (knock you to the floor type bad) you may want to see your doctor to get the prescription strength.
There are some things that do contribute to worse period cramps. These are carbs, caffeine, and (unfortunately for most women) chocolate. It may be difficult but if you can avoid the intake of foods or beverages that contain these things, your cramps may begin to let up.
Also try getting more calcium and Vitamin D. You can do this by eating more foods which contain them or taking a supplement. You need both because they help absorb each other. Vitamin D and calcium help your muscles function better, which should help decrease your cramps.
Of course, if nothing above helps you, or if you are still unsure, you should definitely see your doctor about it.
REFERENCE: http://www.womenshealthmag.com/health/vagina-pain