5 week old not eating as much, sleeping all day please help

So my little baby was 5 weeks on Wednesday, and typically she would eat 3.5-4oz every 3 to 4 hour. Today she has slept All day long, she will wake up to eat but she hardly gets to 2 Oz and she is back to sleep and I can't get her to eat anymore. I'm so worried about my sweet baby girl. I don't know if this is normal. I'm afraid she is going to get dehydrated. Please please help.