Its different for all females. Its an individual thing. Most of mine are good about me messing with their tub or babies.
But some girls get overly maternal (which isn't their fault) and get very protective when they have litters and/or only when they have pups present. When the pups are old enough to walk, that instinct to bite/protect starts going down gradually.
If your girl was tame before, she probably is still tame. Its just the presence of her litter that's putting her on edge.
You can't really avoid the aggression since its instinctual. Just be aware.
But I have noticed that the girls I have personally raised and handled from weaned age don't mind me messing with their babies. I guess they are familiar with me and just know I won't hurt them or their babies.
I've only been bitten twice and those were by tame older girls that I got second hand.
Sent from my DROID RAZR using Tapatalk 2