Express brest milk - supply getting low

Dersheta

My baby is 3 weeks and 4 days old. I feel like my brest milk pumping is already decreasing. Earlier I was able to pump 70 to 100 ml. Now I can hardly get 30 to 40 ml in the same given time in the same pattern of pumping. My brests do feel a little light after feeding my little angle. I am just worried why my pumping supply is getting low.