It could be a false pregnancy....sure sounds like you kept them apart plenty long enough. A female's fertile period is normally between days 9 thru 16.
After a first heat cycle, I found that their teats and vulva do not go completely back to their pre-heat size. The nipples stay a little more prominent and the vulva, though no longer swollen, will be a bit bigger than it was before. My female had her first heat last November & there was a time after, that I too wondered about her being PG, even though I knew she could not be.
For reference now and in the future, this website
Canine Reproduction Video and Book on Breeding and Whelping dogs by Debbie Jensen for dog breeders and Whelping Supplies needed to deliver puppies. is packed will helpful information from breeding to weaning.
Good luck to you! and welcome to YT!