thinking that my boyfriend billy is the dad

Hi,
Welcome to ehealthforum,
It is difficult to say who would have fathered your baby if there was no period in between your two different sexual encounters, as the encounter after period and close to ovulation time would have resulted in pregnancy. If fertile mucus had been present at time of intercourse, sperms can stay alive for 5-7 days and fertilize the egg upon ovulation. After ovulation, egg has only 24-48 hours to get fertilized. The intercourse during your peak fertile days or closer to your ovulation day is likely to result in pregnancy. If your last periods were normal and if your due date (completion of 40 weeks of gestation) is known, then you can count back to know your probable conception date and current gestational age. If your periods are regular, you can also calculate your ovulation date and current gestational age based on the menstrual cycle length and last period date. Ultrasound scan done for dating can also help with knowing your probable conception date. The exact results can only be made out after a Paternity test. Visit your doctor/gynecologist for further assistance and clarification.
Take care.