
Some bitches put everything they have into rearing their pups and this can leave them in a poor condition by the time the pups are weaned that is why it is important that food is increased and puppy kibble given as it has more calories in it, I make sure there is some available 24/7 and then give other tastier things at intervals through the day to help increase their intake.
The pups will be feeding less from Mum now they are being weaned so she won't be burning up so many calories and will gradually gain back her weight and can go back to her normal prepregnancy diet.
One of my bitches came to me when her pups were 12 weeks old, I had met her several times before but she had lost so much weight/coat she looked like a different dog, her normal pre pregnant weight was 8.5-9lbs and she was down to 7.5lbs, I was told she was also very active with the pups which burnt off the calories, with no pups to worry about at home with me she was back up to her normal weight by the time the pups were 6 months old, her coat took a bit longer.
She went on to have 2 litters for me, nearly 2yrs and 3yrs later, and although she lost a bit of weight I managed to keep on top of it, she was more laid back with the pups and let the other 2 adults help out when it came to playing/teaching them things so that will have helped too.
So provided she doesn't appear ill then I wouldn't worry.