Take what she's said on board, BUT make this decision for yourself. Use your gut instinct, research this topic a little more if necessary, and utilise all the support available to you. Some babies just won't gain as much as quick as other babies, there's nothing wrong with that. If you feel Formula top-ups are beneficial then do it.

Have faith in yourself, and work on your confidence, she's your baby and you're her mum, YOU have the power to make this decision.

Hope it all smoothes out quickly and this is just a distant memory soon enough.
xx