i mated my staff bitch for the 1st time, she should be 50 days now her teats are slightly swollen but she doesnt seem to have put on much weight, been really fussy with her food..will not eat dog food at all. should she be showing by now?

she may not show much if he is having a small litter. 

Guessed you would be better off taking her to the vet and getting her checked out, she may be having a phantom or a small litter.