My wife had the identical experience. The first 10-14 days following completion of radiation treatments were her hardest days (physically and mentally) that she has experienced to date.
By day 16, when we had a scheduled follow-up with her radiation oncologist (“just to see how you’re doing”), she had turned the corner and was doing/eating/feeling much better.
She continued to improve steadily, and since then (for the last month or so) has been living and feeling completely normally and comfortably. She’s 8 weeks post-radiation treatments now and awaiting her surgery while feeling and doing just fine.
Following radiation treatments, she relied on IV fluids twice a week (at chemo clinic), oxycodone, gabapentin, and all the other standard meds that I’m sure you’ve had prescribed for nausea, constipation, and esophageal pain. She had a low point or two when we thought it was time for a feeding tube, but I think now that we were wrong.
She wouldn’t have made it through that tough period without IV fluid twice weekly as a supplement to her Ensure-only diet during that 1-2 weeks following radiation. She had a difficult time even swallowing enough Ensure.

In retrospect, I think her esophagitis continued to increase following completion of the radiation treatments. My impression is that the radiation oncologist and staff fully expected that but didn’t really prepare us to expect her condition to worsen for 1-2 weeks before steadily improving.
Based on my wife’s very difficult post-radiation experience (and how well she has been feeling for the last month or so) as she awaits her surgery, I have to say, Hang in there! Take advantage of all the meds and fluids available. Look to turn a corner sometime between 12-16 days post-radiation.
These days, my wife is eating whatever she wants, eliminating most meds, gaining her weight back slowly, and walking for exercise.