Was it fertilization?

I had sex with my man on the third day after my periods. On the fourth day, I got a lot of pain on the lower left side of my abdomen.

I thought it was ovulation, but it was more painful than ovulation pain.

The pain decreased on the sixth day. On the seventh day, I went to hospital and I was told it may be an infection.

The pain went away that day without any medication.

Was it fertilization taking place? Does fertilization hurt? What else could it be?