If i'm going to have a cat scan w/o and then w/ contrast, why would i have to drink barium sulfate the night before? wouldn't that mean it's only w/?

Contrast defined: Contrast can mean intravenous (IV) to help define the blood vessels, or contrast like you are talking about where you ingest a solution that outlines parts of your body - in your case barium to outline your GI tract. If they want to see the parts of your GI tract further down the line, you need to drink it night before. The with/without in your case is likely referring to the IV contrast.

See answer: Cat scans are often performed with both oral contrast (which you drink before the exam) to opacify the GI tract and IV contrast to opacify the vascular structures and enhance various soft tissues.