Sleep all day feed all night

My baby is 3 weeks old and he literally keeps me awake ALL NIGHT! In the day he is a dream he feeds well, sleeps quite a lot and is generally an easy baby. At night he wants to feed all the time but he gets really frustrated, punching my boobs and he is crying whilst feeding. I don't understand as we have no problems with feeding in the day. The last couple of days he has settled once he has done a poo, but it's hard to know what's wrong as he is constantly searching for my boob! Help! Any advice or anyone in a similar situation?