Even if you're not seeing blood, you could still be bleeding and you may be becoming anemic. Have you had your iron levels checked lately? Anemia can certainly make you weak/lightheaded.
Like Sherry, I also have very low blood pressure (both numbers below 100) and that can certainly contribute, too.
Katie, 30. Chicago 'burbs.
DX: Pancolitis as of 9/08 (Proctitis as of 1/08)
Current Treament: Spinach/sunflower & Low-residue diet.
Previous Treatment: Canasa suppositories, Cortifoam, Prednisone, Colazal, Symex DuoTabs, Rowasa, Bentyl, Cortenema, Asacol, Iron, Zoloft, Acidophilus.
Status: Near total remission