It's very unlikely, but it's possible. Here's the crucial question: Did his penis touch the outside of your vagina? Men do have a few drops of pre-ejaculate at the tip of their penises that does contain sperm. So it's possible that sperm from that fluid can enter the vagina, swim up toward the uterus and fallopian tubes, and fertilize an egg. But, like I said, it's just not that likely.
Also, since it was the 18th and you expected your period to start 3 days later, it was very late in your cycle and doubtful that you would have been ovulating that late. We usually ovulate in the middle of our cycle, about 14 days before we start our period.
Does that help? My guess is that you're just a few days late in starting your period. Have you been under a lot of stress lately? Have you started or stopped any medication or had any big changes in diet, exercise or etc? Those can all affect our cycles as well.