I experienced the same thing as you. I got through all the tests, scans, 11 chemo sessions, neuropathy, surgery and 25 rounds of radiation before the wheels fell off.
When I went in for the first of the final five boost radiations (30 in total), I had a complete breakdown. It shocked me and I know they were rattled as I had been a "model" patient - their words.
I was screaming to get off the table, uncontrollable crying, shaking and gasping for air and trying to squirm out from under the friggin machine, not advisable lol.
Only valium got me through the final five. I thought it was just my claustrophobia but I soon realised it was more serious and I needed help. You are NOT crazy.
I was referred to a psychologist by my GP and boy has she helped me a lot. I have been diagnosed with PTSD, not only from the cancer treatment but so many other events I have experienced in my life. I thought I was "over" them but they all came to the surface like an explosion. I guess everyone has a breaking point.
Give yourself a pat on the back, let out the emotions and gradually you will feel better. I'm not there yet but 6 weeks later, Im a lot better.