It's probably a Braxton-Hicks contraction, which is nothing to worry about, it's just your uterus practicing for labour. And yeah, they can feel kind of crampy sometimes (they did for me in my first and fourth pregnancies, in my second and third I barely noticed them). Braxton-Hicks are usually irregular and don't increase in strengh, if they start to become stronger, or more regular, or if you have any spotting or unusual discharge, call your Doctor or Midwife right away. (21 weeks is early to be feeling them, but some women do. We're all different.)
I was told that to deal with Braxton-Hicks, the best thing to do was have a big drink of water (because dehydration can trigger them) and then do the opposite of what I was doing. If I was moving around a lot to sit down and relax, if I was relaxing, to get up and move. It usually worked.
Good luck with your pregnancy!
P.S. Thank heavens for the internet, when I was pregnant with my oldest 19 years ago and was worried for exactly the reason you are, I called my Mother at 3 in the morning. Scared the heck out of her until she realized it was just a Braxton-Hicks and then she had a really hard time not being mad at me. A really, really hard time not being mad at me.
answered 16 Feb '11, 03:26
I don't have much experience with menstrual-like pains beyond the first trimester (which I was told was due to the uterus preparing itself to grow larger in order to "house" the baby), but this article outlines some instances in which you might feel 'menstrual-type cramps':
However, it is important to note that,
answered 17 Feb '11, 12:27