BF - remember that each of us is different. So there's really no way to predict with certainty what you'll experience. I recommend you do a search for various terms - radiation, side effects, etc. and you'll find a wealth of input from many OCF members.
I think for many of us things started to tick up side effect wise toward the end of the second week. I had a ton of mouth sores, heavy mucositis, burned skin and plenty of pain. Also crushing fatigue. I did not get a PEG and wished by the end of the time that I had. So if you haven't done so and are still considering it, please consider it some more!
As your symptoms increase to whatever degree, continue to ask specific questions - both of your medical team and here. You will surely get a helpful answer from someone who's been through exactly what you're feeling.
You'll get through it. We're here to help you. Courage!