Sorry to hear about your stomach problems. Last fall, I suffered from severe nausea for about 2 months. How long has this been going on?
I never actually threw up, but was on the verge of it most of the time. If you're only drinking protein drinks (such as Ensure) I wouldn't worry about the constipation. It will come out eventually (but it may not be very nice).
Did you experience the fatigue/weakness before the stomach problems started? I still haven't been diagnosed, but a whole slew of things have been ruled out. The symptoms sound like maybe Crohn's or colitis. Depending on the location of the pain, can you rule out a hernia of some sort or appendicitis? The pain will hopefully be the clue that will solve this.
You need to see a doctor about your symptoms ASAP. Please dont wait any longer.