
Are you sure it's a different bitch? Could the photos of Mum have been old ones and maybe her colour has changed or she has lost weight/coat or vice versa? My own breed change colour all the time, a dog that is gold at a year old could go light fawn or deep red as adult.
Assuming you're sure they're different then it does sound very suspicious. There are of course innocent reasons why this could happen, say the breeder has other bitches that love pups and they've just popped in to the whelping box to mother them while Mum is taking a break, but then the breeder should have said something like 'here's granny such and such with the pups'. Also, perhaps the breeder is just not good with technology and sent you the wrong Mum photos, I have a friend who's always mis labelling his dogs on facebook, and having everyone else correct him

I'd not worry till you get a reply from the breeder, if your gut tells you the reply is nonsense I would ask if you can come and see the pups.