You are right to listen to yourself. You can't force her to have more if she doesn't want it. You are so aware of her health and her overall well-being, i'm sure you'll know when/if you need to seek further medical help.
I would tend to agree about the GP telling you to start formula for no really good reason. Heck, I took DS2 to a GP for thrush and she told me that I should think about it and he weighs 6.5kgs!