Hi there,
I also had a low grade BC, and had a lumpectomy and radiation. Remember your body is still healing from the surgery ( the 6 weeks is a basic healing, it takes months to really recover). Add that to the stress of the diagnosis and living with the treatment and its not surprising I guess that we need more rest. Radiation adds to this and
I also became tired early during radiation and also had some feelings of nausea almost like morning sickness which the doctors said was very unusual, but which was able to be easily treated. I found I needed to rest a little most days, even just a sit down or lay down for 30 minutes helped during the weeks of radiation treatment . However, 8 months on I still have days where I am tired and need to have a rest, not every day but maybe one or two days a week. Surprisingly, because it seems counter-inuitive, a 10 minute walk often picks me up if I can drag myself out on those days. There is some information on cancer fatigue which might be useful for you. Good luck with your treatment, and be kind to yourself