They can be on and off ur whole pregnancy. Its from your uterus growing. Im 28 wks and still get crampy. As long as ur not bleeding with the cramps they are normal and will differ in every pregnancy. With my first pregnancy i had horrible cramps all the time. The beginning was the worst the 2nd trimester was a lot better and the 3rd trimester they came back.
usually cramps like your talking about tend to be felt around the 5 wk mark, they usually come and go and are from the uterus stretching, your uterus can also feel full like your period is about to start.... not all women will experience them, if they are accompanied with any type of bleeding you should let your doctor know
i got them from week 1 through week 8 because all the legiments and the implantation hurt so bad. but it did get better but i think the more pregnancies you have the less pain you have because this is really my 4th pregnancy but my 3rd healthy pregnancy and the pain in the 2nd trimester is almost pain free=).