Help!

Hannah
My baby is 3 weeks old and up until a few nights ago, she had been sleeping very well through the night. Only waking up twice: around 1am and going back to sleep at around 2am and then waking up again at around 6am and going back to sleep at around 7am. 
She sleeps great through the day and my pedi said make sure to wake her from her deep sleep during the day every 2-3 hours so she doesn't get her days and nights confused. 
Now, during the night she's been waking up at 12am and screaming and crying and fighting her sleep and not falling back asleep until after 3am and she HAS to be on my chest. She's not hungry either and I've tried gas drops and gripe water and it doesn't help. 
Any tips on getting her to sleep longer through the night!?