This is more likely the result of accumulated assaults on the body finally taking place and manifesting themselves, perhaps combined with natural anxiety over the beginning of new treatments. Radiation sickness does cause nausea, but after one treatment, I find that unlikely as the cause. There is no reason that I can think of for IMRT radiation to have this effectso early on. Though clearly anyone that has been through radiation treatments will tell you that lack of energy and lethargy is a common side effect starting about week 2-3. Anti nausea meds are certainly necessary if the problem continues, partunlarly during hte treatments. I was un bolted from my mask more than once towards the end of my radiation treatments to fill up a bucket next to the machine. Make sure that your doctors are on top of providing the appropriate meds, but expect them to also create more sleepiness.