Yes! And after mating they are usually very randy and will go looking for a dog. Double conceptions happen quite often, so if you have paid a stud fee or have had your bitch mated to a dog of your choice (rather than her choice!) you need to keep her away from dogs for quite a while yet. If you mate a bitch on say the 12 day of season I would be careful for at least another 3 weeks.
Unfortunately I have a male & female (you may have seen my post a few weeks ago that I found them both) anyway the vet said the female had just come out of season so we thought we had ages before we had to do anything (don't know where I would stand legally if I neuter them as they are not ours until six months have passed) but yesterday I caught them at it and they have not stopped since, we decided there was no point separating them as it is probably too late now!
Are they mating and tying or is he just mounting her? If they are actually tying I would be inclined to separate them. If she has been out of season for a while its possible she has some sort of infection. Might be worth another word with your vet. I don't really know all the circumstances, so may be on the wrong track here.