Could radiation therapy make my breast feel warm months later?

No: Usually not. The warmth of breast months out from radiation is more suggestive of an infection, or inflammation. I would recommend to followup with your surgeon for your oncologist if it does not resolve. Especially if it is associated with redness, fever, tenderness.

No: Months later skin reactions should have gone back to normal. However some patients may develop mastitis. In this case the breast will look red and warm and obviously different than the other side. If there are obvious differences then make sure you see your doctor.