Baby does not sleep during the day

Need some advice here. Baby is 3 weeks and as of a few days ago she is unable to fall asleep during the day. At night she will sleep at 3 hour increments and gets about 9 hours of sleep that way. We make sure the change her diaper at least every 2 hours and feed her every 2-3 hours. She looks so tired after each feed and we hold/rock her until she falls asleep which lately is not happening. You can see her drifting in and out of sleep but as soon as we put her down she is up.

What am I doing wrong???