HELP!

Has anyone experienced septic or any other kind of infection in there newborn? My baby is a week old and has suddenly started eating less and is wanting to sleep more. She is a premie and was told she would want to sleep a lot and would have to wake her up to feed her. Usually we are able to find something to wake her up enough to eat about 30-50ml every 3-4 hours but now she is only eating about 20ml every 4 hours and is taking about an hour and a half to feed. She is a formula baby due to my milk never coming in!