I had sex with my boyfriend and after he ejaculated inside of me I suddenly got some cramping/pain on the left side lower abdomen. What could that be?

Could be coincidenta. L or the effect of the ejaculate;which has in substances,prostaglandins that could cause uterine contractions.
Well . If it happens soon after sex it could be a physical opening of the penis against your cervix or stretching the ligaments to suspend your uterus or could be a.m. hormonal chemical reactions to the sperm. If this happens more than a few hours after sex or lasts more in a few hours seek medical attention. For specific diagnosis and treatment See obstetrician gynecologist or family doctor.