If you havn't been eating many raw veggies, the added roughage can cause bowl upset. What may helo if that's the case, is to cook veggies for a little while, and keep raw amounts small until you have had time to adjust.

You might also look into if you are lactose intolerent. Are you eating cheese or cream where before you weren't really having any dairy at all? It could be another food intolerence besides lactose.

As you have had surgury recently and probably quite a few antibiotice, you might want to read the chapters in the book on yeast and candidia over growth. The antibiotics may of killed off healthy bacteria in your intestines, letting the bad bacterias grow abundant. If this is the case it can effect weight loss, cause irritable bowels, make food intolerences, and other problems.