Hi,
It's probably due to your fluctuating hormones right now; lots of women experience crazy side effects from hormone changes during breastfeeding. Try cutting back on caffeine, processed sugars, and high acidity foods in your daily diet to see if it helps. Taking short, cool showers throughout the day will help you feel more comfortable. Keep frozen yogurt or natural juice pops nearby when you breastfeed; they make great snacks that will help you stay cool :)