It takes awhile to ramp your intake up to required levels. You've just had major surgery, even if you only have a few bandages to show for it. There's a lot of post-op swelling to consider, which is why they start you slow and ease you back into eating again.
Follow the rules, take it easy, don't stress yourself, and good things will happen. You need to first get your fluid intake up to snuff, then work on your protein. If you do your part, your body will reward you.
Good luck!
CT