Having menstral cramps only on the left side what could be the reason to this?

Referred pain: Possibly you are subjectively experiencing the menstrual cramping on the left side, and if you normally have no pain when you are not menstruating, i wouldn't worry about this if it is the first time it has occurred. You could have had a cyst on the ovary that ruptured, you could have uterine fibroids that are affected by hormonal change.