Everyone is different and every pregnancy is different. I had cramps during my first pregnancy which felt a lot like period cramps, but I did not have cramps during my other two pregnancies. They usually disappear after the third month (12 weeks.)
If it is light cramping and not accompanied by bleeding, they are normal and are caused by the uterus expanding and stretching to make space. If they are severe and/or accompanied by bleeding, it is...
Cramps can begin as soon as implantation occurs and can go on later in the pregnancy as well. It is completely normal unless heavy bleeding occurs as a result.
Cramps can occur at anytime, pretty much from shortly after conception (implantation pains) and then throughout pregnancy. They are not normally serious unless accompanied by other symptoms...