MLC,
The difficulty is the fatigue makes you want to lounge more which slows everything down in the body and makes you more tired. Without breaking that cycle by sitting up at least throughout the day and avoiding naps as much as possible, it starts affecting sleep patterns and that slows healing, increases pain and also contributes to many of the neurological deficits created from chemo and/or rads.
The important thing is keep the doctors in the loop just as you are. If you believe it is beyond just being whooped from the treatment, you need to be a bit more stern with the medical team. If it troubles you, make it trouble them as well, or nothing will get addressed. It's not the time to be timid.
As David says, get the attention now, it will get tougher with more rads.