I have never known a litter to be born so early naturally and be ok , so I would imagine that the little hussy had been busy with a young man before you realised. Are the puppies ok, and feeding well? Most bitches give birth between 59 and 65 days, usually day 63.
I have asked on my doggy group and the youngest surviving premature puppies were born ten days early (53 days), any earlier and they were either born dead or died soon after birth. Those surviving took a lot of handrearing and feeding every two hours and TLC to get them up to their normal weight.