Firstly, is she looking after the pups? ie, is she feeding and cleaning them? If they are healthy and putting on weight then that part I wouldn't worry about. If she isn't looking after them then I would start to worry. You may have to hand feed rear them :(
A lot of bitches go off their normal food for a time. Try her with some raw mince meat. I use chicken to start with, or beef mince if necessary as it is smellier ;) Tinned rice is also a good thing to try to get them to eat again, but not too much. She really needs to eat to make the milk for the pups.
She is obviously a dog that is very happy with her pack mates, and hasn't realised that she is supposed to look after the pups. I had one bitch that fed, cleaned and then brought her daughter to me to look after. She had done the 'doggy' bit, but the pup was mine after that. There again, she only had one pup.

I agree, as well as they are putting on weight, she is cleaning & feeding them, and goes back to check on them every so often, it's not a problem. My girl has been leaving hers for hours at a time right from them being a few days old, but if you go in there and pick one up, or if she hears them squeak, she's straight back in to check on them. :-)
I had my first litter recently, so not that experienced, but I was suprised at how relaxed and laid back about it all my bitch was.
She had a litter of 9, and used to stay in the whelping box, including to be fed, for the first three days, but at nights would sleep next to the wehlping box by me, (she couldn't leave the room, I shut the door and slept in there with them) only going in to feed or clean them, and checking them if they made a noise. I have no other dogs, but she continued to be very laid back, even wanting to go for walks. She was a very good mother though, playing with the pups, although maybe a little too tolerant of them (they used to love all pulling her tail when she went in to feed them!). She continued to feed and clean them up until about 9 weeks. We have one pup still here until Tuesday, as the new family have been on holiday over Christmas and although Mum has now stopped feeding, she is still excellent with the pup, and will clean up after her if allowed. She also waits until pup has finished eating before she eats, despite having seperate bowls.
I went by the weight gain of the pups, and how they were doing in general as to how good a job mum was doing, so, in short, I would agree with the others. If pups are doign well, no need to worry.