What could cause somebody to always feel sick after they eat, and how can they prevent the problem?

That's hard to say. I was always sick after eating until I found out that I have Celiac. I now stay away from all glutan and am feeling much better. However, it has taken 2 1/2 years to heal my insides of the damage done. In fact, I'm still healing. I understand that only about 3% of Celiacs ever get diagnosed....Even after being diagnosed, I had to be very aware of everything that I ate...as to how I would react to it. In this way, I have weeded out several things from my diet that cause problems, like milk products (except kefir) cooked tomatoes, etc. even though they don't contain glutan.

It could be many different things ... allergies, ulcers, kidney stones, stress ... who knows. You would have to either stop eating certain foods or speak with your doctor about medication that could counteract whatever is upsetting you.