Usually baby gain birth weight by 7-10 days if delivery in by 9-10 month pregnancy. If preterm can take up to 2 -3 weeks. Give adequate breastfeeding. Give atleast 8-10 sessions of breastfeeding. And each session feed for atleast 10-15 minutes . Last part of breast milk is also necessary so try to feed complete with one breast and than shift to other breast. If baby is sleeping 18-20 hours in 24 hour and passing 6-7 times urine over 24, that would be adequate for her. You should take proper sleep, nutrition and keep hydrated yourself. Add some fresh fruits in your diet.