What could be the cause of tenderness in the lower abdomen and cramping?

There could be many reasons for your discomfort. I think before a diagnosis could be made by a doctor, or a suggestive diagnosis could be made from us on the boards we would need more info.
1. Do you have loose stools or constipation?
2. Do you have any rectal bleeding?
3. Have you lost any weight recently?
4. Do you have any food triggers?

These answers could help to determine if you have something like Ulcerative Colitis, Crohn's disease, IBS, lactose intolerance, or something else which could cause cramping and tenderness. Also, are you male or female? Lower abdominal pain and tenderness could be gynecological or a hernia too.