I had a similar thing happen where from leaving hospital C had lost 70g at 1 week 5 days and then the next weigh in the following week he had put on 540g!
I'm sus that it was a dodgy set of scales because that amount in a week is almost unbelievable. The MCHN was sus on them too and it was the same nurse who did both weigh ins.

How is her output? I'd be taking lots of notes about her nappies to keep tabs because if she's wetting and pooing lots then it is getting in there!