My stomach is always weak after I eat and then it cramps bad to,but its not time for my period to come on what could be the problem ?

Your stomach is above your belly button, your uterus and ovaries pretty far below the stomach. Granted you can have digestive discomfort with your periods, but we need more information.

What do you mean by weak stomach? Diarrhea? Churning, nausea, vomiting, gas?
By cramping do you mean a real twisted muscle cramp feeling, or just pain? Near your stomach or where?
Is it a cycle that follows your menstrual cycle? Is your period painful or crampy?
What is your typical diet like?
Have you seen a doctor?
How old are you? Are on on birth control? Are you active and mostly healthy otherwise?