This must be a worry for your mum coralee
Both of my girls go off their food when in season...so this could be another reason for Calle not eating
I would ask your mum to contact the rescue centre to find out how many days she was kennelled with the staffy, from this she may be able to work out if Calle was at her fertile time of the season.
Could be from days 7 - 16 approx.
If your mum is really concerned that Calle may be pregnant, perhaps it would be advisable to get her scanned
If she does have puppies it would be best to be prepared beforehand. I will keep my fingers crossed that this isn't the case & its just Calle gone off her food temporarily.