Hi! So the the first day of my last period was nov.12 and it ended on nov.17. We had unprotected sex on nov.18. Could I be pregnant?

Unlikely. With short cycles & long-living sperm, it could be possible but unlikely. With normal 28-30 day cycles, your exposure would require sperm to survive for a week. For women using the rhythm method for birth control, you performed it perfectly. Let's just hope you didn't ovulate early & check a upt if you miss your next cycle. Good luck & consider more effective birth control for next time.