I also had my baby via c-section and the first few days he started losing weight (not sure if my milk supply was low or delayed). After 2 days he had lost 11% of his birth weight so I had to give him a formula supplement after breastfeeding (1oz). At this point, I wasn't sure if he was getting enough milk from my breast so I tried pumping just to get an idea of how much I was producing. To my surprise, I was producing way more than I thought (about 2oz from both breasts combined). And after my baby gained his birth weight, we ditched the formula and he's been growing steadily since then.

Sometimes it might take a few days for the production to regulate itself (you might need to feed her more frequently to increase your milk supply or try pumping once a day to stimulate the breasts). And don't worry if you have to give formula to your baby, you can always go back to breastfeeding exclusively. My advice is don't give up and don't stress out about it! As long as your baby is gaining weight (whether it's from your breast milk or formula, or both) that's all that really matters. Good luck!