Brief Answer:
Possibly because of taking
Novelon.
Detailed Answer:
Hello,
Thanks for posting your query.
If her period was supposed to come now and you had intercourse 15 days ago, chances of conception are a bit high particularly because you did not use any protection like condom and your fiance feels that you ejaculated inside the vagina too.
If she was using Novelon from her previous period then the chances of
pregnancy would have been quite significantly low, almost nil, but since she was not taking them since the start of her previous period, chances of ovulation are quite high, and moreover you had intercourse right in the middle of the cycle when ovulation happens.
Considering all this, I would suggest you to do a home pregnancy test after one week from now, it takes some time for the test to become positive, and if the test comes positive, you can decide on future course of action as per your needs.
As for the scanty and watery menstrual flow, it is quite possible that Novelon is responsible for this, taking and stopping
oral contraceptive pills can cause menstrual irregularities, that is not a big deal.
I hope this answers your question.
Regards.