How to settle my baby? please help im desperate?

im the mum of a 3 week old daughter. she sleeps a lot during the day and is difficult to wake.. although she does get a few hours of awake play time during the afternoon. around 5pm most afternoons she gets very irritable and cranky and this goes for about half an hour to an hour. I give her a feed (usually 5oz) because she's a hungry baby, burp her in the middle of the feed and after, put her down and she sleeps until about 1am where she wakes for a feed and change.. but is impossible to get back to sleep no matter how much i rock her, feed her, burp her, change her.. nothing works! not even her musical mobie which plays lullabies.. im at my wits end as im not getting much sleep during the night... i know its to be expected but she's impossible to settle during the night.. she might sleep for 5 minutes but as soon as im back in bed with the blankets over me, she starts crying again. HELP ME!!