Since you are under medical care, you probably need to talk this over with your surgeon.  Be prepared for the idea that you will get a lot of input that is not quite correct.  Doctors are not trained in nutrition, and by and large, they are not well informed as to a vegan diet.  It is going to be more productive to listen to the doctor's concerns than to argue because some doctors are well informed and at any rate, your doctor should be more familiar with your particular health concerns. Take their advice into consideration. I suspect you are on target with the giving up dairy because it is an irritant for a great many people.

If you have stomach bleeding, you are going to need to be careful of acidic foods which will irritate your stomach...tomatoes, citrus, fruit juices, which you have probably already been told.  Avoid all fried foods.  Because you have developed anemia you'll need to research on your own for iron-rich vegan foods such as dark leafy greens and lentils. 

My son who was an Angus Beef only guy and ate mostly this food, has recently converted to being a vegetarian and is looking at veganism.  He eats no dairy, but does have the occasional egg.  He has noted that one cannot simply give up meat and continue to live on potato chips and green salad because they would get very sick.  Research carefully.  This site has some excellent information which you can access from the yellow bar to the left on this page.  Read, read and read and go carefully.  Be educated so that you improve your health, and not make matters worse.