So the major surgery was probably a cruciate ligament operation was it ?
This in itself is months of dedicated recuperation to get this right. She would still be limping after 5 weeks. I nurse all cruciates back for our rotties in rescue, and have nursed back about 12 over the years plus given recuperation advice on others.
However, if the dog has gone off food then the vet should be checking for infection here. It is very possible. Feel the leg and see if it is really hot.
You are showing up as UK which means that the American Staffie is actually a banned breed so not sure whether your vet is aware of this and being cautious. He should not be saying "show her heaps of love and attention".... NEVER heard a vet offer that advice for getting over major surgery, and cruciate if it is that. He should be checking for joint infection and then giving you guided recupertaion advice such as lead walks in the garden only for wees and then building very very slowly such as 5 times 2 mins per day for a week and then 5 times 3 mins per day etc....
A cruciate ligament op will take about 6 months in total and even then you have to be careful.
Add - totally with Moondog on this especially regarding the cruciate op.
There is one other possibility here and I have seen it happen, and have nursed a dog through it, and that is if you failed to give her the proper attention when she'd just had the operation. If you left her offlead to run down the garden then it could of gone again.
This actually happened to a dog in rescue that wasn't given correct post op care. Two months later he was with me to help him, and I knew it wasn't right. Got him back to the vet and it had in fact completely gone again.
A slight limp would still be there at 5 weeks though.