
Quite normal, just increase her food and try and keep her away from them while they eat and an hour after she is fed.
To be honest if she is such a good Mum, why fight it as long as everyone is getting enough to eat. If pups are gaining well,a dn are well covered and Mum isn't too thin.
With my most recent two litters, Myka is the Mum of the world type and she was still allowing full access and topping up after every meal and in between until pups were 7 weeks,a dn then one day decided that was it (though of course I expect the supply was going down as they were eating like horses since 3 weeks of the four meals a day I was giving them.
Inka did what most of mine quite suddenly reducing the number and duration of milk feeds at around 5 weeks. All my girls continue with the odd comfort suck (literally seconds) until pups go to new homes or until around 10 weeks if still here. By 11 - 2 Weeks my girls are dry.
I always add puppy milk to the pups food until around 6 or 6 weeks. Basically for as long as bitch still gives milk feeds.