If she is having plenty of wet nappies etc then let her sleep if you think she needs it. She will wake if she is hungry and providing that you are responding to hungry cues she will be fine. She may need to make up for it during the day, so make sure there is more than she normally takes in her bottles and again, follow her hunger cues.
She may be not so hungry because she is feeling unwell, if mine fed less due to illness they generally made up for it when they were better!
Hope she's feeling better soon.
Laura