boobs deflated?

So my girl was born 6 days ago and breastfeeding is going awesome. My milk came in after 2 days and I had tons of it, my boobs were HUGE. By day 4 they were doubled in size and obviously full of milk, now on day 6 they've shrunk back down to smaller than they were when I gave birth. Why??? Is this normal? She is still eating for the same amount of time and doesn't seem like she's hungry after. I can still easily express milk. But my boobs deflated. Is this bad? Is my supply getting smaller? Is there anything I can do to fix it if so? I plan to call the lactation nurse today but would like to know if y'all could give me some advice or if anyone has had the same thing happen to them and their supply not been affected? Thanks!