I think Chrys' comment is correct.
Also, it takes a while for the baby to find and settle into her own personal rhythm, and there is lots of variation between babies. You're probably right that 13 hours per day is on the low side but that doesn't automatically make it something to worry over. Give it two more weeks and you'll see a sleep pattern.
You can also try to introduce your own schedule, for instance do the sleep/feeding/diaper-change routine on a 3- or 4-hour basis regardless of whether she's crying for it or not. With luck, this could help her into an evenly spaced rhythm that you can plan into your days and nights with reasonable precision. Some kids accept a rhythm like that, some don't. Wait and see.