it's mostly likely from the IV fluids they gave you in the hospital. It's basically salt water, which a lot of people retain. I've heard of people leaving the hospital weighing up to 10 lbs more than when they went in! It does take a few days to work it's way out of your system, so hang on. You should start dropping weight very soon.
as for protein shakes, I never had trouble with drinking things from day 1. I was worried about it because we were told we'd only be able to drink a little at a time, but the PA at my clinic told me not to worry about it - that my stomach would tell me if I was drinking too fast or too much.