Unfortunately all dogs vary in their heat cycles & even though she may have been in her cycle for the average time (approx 3 weeks) it's obvious since she allowed mating that for whatever reason she was still in her fertile period when your male got to her. Since she allowed him to mate with her there's a strong chance pregnancy will be a result. Of course, one can always hope it didn't take but that's only a best case scenario. I would definitely advise you keep them separated for at least another week or so to ensure he isn't able to mate her again. Considering her age (I'm assuming she's between 6 to 9 months since you stated this was her first heat) you should seek your vet's advice on whether or not to terminate the pregnancy. You may also want to consider having her spayed (which can be done at the same time if you choose to terminate the pregnancy) so that this scenario isn't repeated. At such a young age she may have great difficulty in caring properly for a litter and the health risks are far greater for her. If she should need a C-section, for example, it can easily cost up-wards of $1000. If you would like any additional information feel free to contact me.