It's a bit unusual for me, but one of my girls did that:
10/31 3622g
11/20 4137g
I'm a bit shocked for that more than 500g less than 1 months..
Feeding weekly 150-180g F/T rats, she never refused a meal.
Pairing since 10/10.
Last defecating 11/10
Have anyone else with similar experiences?