I think I would be leaning towards what the rad Doctor says in this situation. They are MUCH more familiar with the side effects and timing of them than someone outside of the specialty.

It is VERY eary for you to be having severe side effects. While the sores in the mouth aren't that unusual ( I began developing them at 5 days) the rest certainly sounds more like an infection than anything. If it's a flu, its viral, and antibiotics wouldn't help anyway.

Once you start to really feel the effects of rad, I'm afraid you won't be trying to guess what they are...they are profound, and miserable. I have never been as ill as I was during radiation.

I hope you can get feeling a bit better quickly. The radiation is tough enough without a flu or some other bug adding to it
Wayne