Milk flowing really fast for 3-5 mins

My son is 2 weeks old and eats about 3-4oz each time. I am doing a mix of bottle feeding him breastmilk and actually feeding him from my breast. When I manually pump I can get up to 3-5oz in one pump session, when i use my spectra pump I double pump & get about 1.5oz out of each breast. Is this normal? In the beginning of every pumping manual or not I get so much milk out so quickly it squirts out for 30 seconds each time but then after about 5 minutes of pumping it seems like I just run dry. Is my supply lowering??