Hi!
If you were my patient, I would tell you not to worry about that encounter--it is not enough to result in pregnancy unless he did ejaculate in or near the vagina. If it has been less that 120 hours since the encounter, you can take emergency contraception (available at most major pharmacies without a prescription) but I think that is probably overkill. You could take a pregnancy test in two weeks and if negative, know that everything is OK.
You should probably see your doctor and get oral contraceptives, both for birth control, but more importantly to regulate your cycles.
Good luck!
Dr B