My dog had pups 12 weeks ago is it normal for her to lose so much weight and for her to go off her food a bit

Those pups are taking a lot out of mom; she is giving all her attention to caring and feeding for them. She will come back to her food when all is well with the pups.

You didn't say how much weight she lost. As far as food goes they usually have to be made to leave the pups to eat at first since her focus is totally on the pups. You have to make her eat. At 12 weeks she should be over all that, the puppies already weaned and things should be pretty much back to normal, only about a month to go and the pups are ready to leave. I would suggest if at this point she is continuing to lose weight and not eating you need to get her checked by the vet.