Confused and FRUSTRATED (Long description)

So my baby at birth was in the 77th percentile for weight and then ever since she was 1 month old till now (she's exactly 5 Months) has followed the 50th percentile. My earlier Pediatrician thinks she was doing great! But he fell ill and I was referred to another Pediatrician and when I visited her she seems to find all sorts of 'possible' problems with my LO starting with her weight gain which she thinks is inadequate and wants my EXCLUSIVELY breastfed baby to be supplemented with formula... she is calculating LO's per day weight gain and thinks it's wayyy too low but LO's monthly weight is exactly on the 50th percentile. Baby has more than adequate pee nappies though her stools are once in a couple of days (started with once in 3-4 days to now once in 9-10 days). Now I'm just CONFUSED. Is my LO's weight okay or is she not getting enough milk???

1) very good number of wet diapers

2) active playful baby

3) reaching her milestones

4) does not cry from hunger ever. But does suck her fingers very often and vigorously.... so is that a way of her to satisfy/quieten herself when she's hungry???

5) since 3.5months on, her weight gain isn't as much as what it was initially. But I thought that's normal. Also she's very active, Kicking her legs allll the time, reaching out with her hands, rolling over from back to tummy etc. So is it this that - she may be getting milk but not showing gains as she's spending those calories being so active???

I'm SO CONFUSED and this Doctor is ONLY adding to my confusion without actually resolving it.... what exactly IS happening??

Does weight gain slow down around 3.5-4 months?

Do I really hv a decline in my breast milksupply (but if nothing is going in, then how is she having a more than good number of wet nappies? Anywhere btw lowest 10 to highest 20, but usually 14-16 pee CLOTH nappies in a 24hour period)

Does this sound like a decrease of my BMilk and does it look like i should start supplementing now????

What do u think ?????