I had it all through chemo. My GP freaked out a few times. When I went back to work it was hot weather. I had to have to 3 litres on board not to feel light headed. I was on herceptin also. It turns out iI had refuced haeart function. Medication did assist and a bteak but I had to stop my treatment early because of it. I got almost 3/4 through. It doesn't always mean heat failure though...just telling you my issues. It was months before I could stand still for any length if time snd not get light headed. Recover I did though.