The nausea and gastrointestinal discomfort felt after radiation and chemotherapy treatments comes from how these therapies work. Cancer results from the abnormal multiplication of cells in the body. Instead of dying and being recycled, some cells go haywire and multiply. In order to stop them from multiplying quickly, chemotherapy aims to block the way these cells use energy, killing them. Radiation, on the other hand, is used to disrupt the cells' DNA, making it useless to the cell for multiplication.
The problem is that the cells that line your gastrointestinal tracts (from mouth to rectum) are also cells that multiply very quickly. Just think about any time you've bitten and drawn blood from the inside of your cheek, or the times you go to the dentist. The bleeding and discomfort stop very quick. This is because these cells multiply quickly and patch up any damage.
You can see where this is going... Chemotherapy and radiation also have an effect on the cells that line your gastrointestinal tract, irritating and/or killing them (though not ALL of them like they do with cancerous cells). This leads to all sorts of issues, including the nausea and other GI symptoms.
Newer cancer therapies are aimed at the specific cancer cells as much as possible, but cells that divide rapidly usually share similar structures that may make the chemotherapy also affect them.
Of course, there are other reasons that nausea and discomfort may occur in cancer patients, including infections resulting from a weakened immune system (white blood cells that help in immunity also multiply quickly), or the emotional stress that the disease places on an individual.
Your health care provider is the best person to consult on available medications to counteract the side-effects of cancer therapy.