need advice from moms with newborns!

My 3 week old just started this daily routine of sleeeping for about half the day (waking up to eat and then going right back to sleep) and then the other half of the day he is fussy, constantly wants to eat even if it's more than 2oz an hour, grunting and will only sleep at the breast. If I put him down he wakes up and won't stop crying until I put him back on the breast even if I'm empty! He has nursed tonight for literally 3 hours straight almost! My nipples are numb! I'm just wondering why he's doing this?