I have a left breast pain 4months, I went to a clinic and underwent mammogram and ultrasound, the result all are normal. And now still in pain. Why?

Mastodynia. Breast pain is usually secondary to hormonal effects on breast tissue or chest wall pain; favoring the former are cyclic changes in the pain; the latter is most common after menopause & is often activity-related. Considering your age, I suggest decreasing caffeine intake and taking vitamin e and oil of primrose.
There are many. Causes for breast region pain, most commonly hormone related in younger women. This wouldn't necessarily show up on a mammo or ultrasound.
? cyclic breast pain. If your breast pain is cyclic, it may be related to changes in hormones during menstrual cycles. It typically is worse before your period and improves when period begins.