Wont sleep in daytime!!?

I have a 5 week old baby. She has just started doing this thing where she wont sleep in the day. I know she is tired coz if i pick her up and hold her she will fall alseep. But as soon as i put her in bed shes wide awake again. She wil just lay there and suck her pacifier but not sleep. is this normal? or should she be sleeping?