Am I at risk for fecal impaction if I don't eat or drink much?

Yes, of course. Stool becomes hard and dry if there is not enough water and fiber in your diet; it's difficult for the colon to move it along, and difficult (and painful) for the hard stool to pass through the anal canal. Chronic use of opiate pain meds can contribute too. In extreme cases hard stool can wear through the intestinal wall. People have become dependent on laxatives rather than eating right.