I remember that happening to me when I was nursing my ds (my first). I had an appointment with my endo for my thyroid and I asked him about it. He said something about breastfeeding bringing on an adrenaline effect warming up the body. I don't know if that's really true or not, or if it was just because he was born in June and we lived in a hot and VERY humid climate at the time. But for what it's worth I think you're normal!