Why do people feel sick after radiotherapy?

It injures your body. Radiation therapy kills cancer cell but can also damage the normal structures of the body. Most people getting radiation complain of fatigue. What other problems there are depend on what body part is treated. The skin gets a sort of bad sun burn. If the stomach is treated, there can be nausea, diarrhea if the colon is treated, and cough if the lung is treated. This all is usually temporary.
Depends. Most commonly, nausea may occur after radiating the stomach. Radiation can cause inflammation in the tissues and nerves within and surrounding the stomach, which can lead to nausea. Some people do get nauseated after radiation to other areas of the body, but this is much less common. Anti-nausea medications are generally very effective.