Post breeding.?

My dog was outside today playing in the yard. Another dog got into the yard and the two of them...ahem...mated. I was unable to stop it before the copulation was complete but since then I brought her inside and fed her and loved on her. But she also has been acting real lethargic and sad. The dog was a good deal bigger than she was but she wasn't bleeding or anything and she wasn't hurt. Is that something that just happens with dogs after they've mated for the first time?