I've had a lump on the left side of my lower abdomen which is not visible when i'm lying down. it hurt particularly after a large meal.

Constipation: Many people don't realize they're constipated &have enough stool accumulate in the rectum &colon that it can be felt or even seen if the person is thin enough. After eating, the colon tries to empty itself, &when the colon tries to squeeze a large amount of stool it can be painful. Of course the only way to know for sure is to have a medical exam, but you might try an enema and laxatives first.