Cramps after boyfriend ejaculated inside on first day of period. Sharp pain on abdominal section.

Hmm. Uterus is already contracting (cramping) during your period. During intercourse, a hormone is released that causes uterine contractions (cramping). Semen also contains chemicals that can cause uterine contractions. So, in all, you experienced somewhat of a perfect storm of uterine contraction/pain. You may want to try (if not contraindicated) NSAIDs like Aleve/Naprosyn or Motrin/Advil for the pain.