Totally normal. DD is 4 months old, admittedly having a growth spurt, but has literally fed every hour today.
Also your baby lost minimal weight (it's normal to lose up to 12% by 3-5 days old), so you are definitely not a failure. It sounds as if you've been one of those rare and phenomenally fortunate women with a baby that feeds well from only a few days old. Well done!
I also get the "She can't be hungry again can she?!" comments. It's hard when you're feeling tired and have the very normal new parent worries about feeding and wanting to do the best for your baby.
The thing to remember though is that MIL will come from a generation not accustomed to breastfeeding and very accustomed to putting babies on 3-4 hour feeding schedules, putting babies to sleep on their fronts, in a different room to mum etc. The UK has some of (possibly the) lowest breastfeeding rates in the world. Formula feeding is hugely entrenched in our culture and even more so in our parents'. That's not a criticism of FF or of parents from a different generation. If anything it's an acknowledgment that our parents' generation were doing the best that they could, but times and knowledge of infant feeding/sleeping etc change. Your MIL needs to respect this.
I was going to suggest you asked DH to have a word, but on reflection I think you should be the one to say something. Be a tiger mother, make a stand, look her in the eye and say to her "I get the feeling you think I'm doing something wrong with the feeding?...You don't?/You think so?... Because I know she's a really healthy weight and the Health Visitor is really pleased with her progress.".
You won't be questioning her experience, but you will be drawing a clear line on who decides what's best for your baby.