Hilarazon​
I am sorry about your diagnosis. My experience with radiation therapy following a lumpectomy in the Left breast was straightforward.
Radiation therapy to the left breast has a slight risk factor associated with ensuring the heart doesn’t get zapped. In my case this involved learning a breathing exercise that had the effect of keeping the heart well out of the line of radiation. The radiation treatment is very precision based and in my case I had nine little dots tattooed on my chest and just under the armpit that provided coordinates so each treatment was exact.
My side effects were similar to those described byarpie​ but I did feel fatigued and made time to rest each day throughout the 15 day treatment. I think my fatigue was partly due to the radiation being on the back of my neo adjuvant chemotherapy, catching COVID and then having surgery so I think it was just that I was probably already pretty depleted by other things.
After the radiation therapy the breast does continue to feel sensitive and warm for a few weeks. I am not sure if it was from the surgery or the radiation but a few months after completing it I found I had a bit of tightness and cording and I consulted a specialist physio who focused on breast cancer care and she was able to provide laser therapy and exercises that helped considerably.
Wishing you all the best with this next phase!