Is it safe for my 6 year old Shih-tzu , that already had 3 Litter's to have another one?

I took in a 6 year old Shih-tzu's , and i have 2 other male dogs, accidentally my male dog started mating with her last night , and i have no clue what to do i called the vet , they said she's going to end up pregnant , but I want other people's opinions and what might happen to her.