If you are not wanting to talk with a doctor about this right now, you might choose to keep a journal about these sensations for the next few months, and notice any changes or trends. A trend could be that your left breast may be more sensitive right before your period (this would be normal). If you discover a trend that your left breast is becoming more and more tender over time, this would be a good reason to call your doctor. You can track things like "degree of pain" on a scale of 0-10, and check each breast each morning/night for the next 60 days (over a few cycles).
If you have any additional symptoms that concern you, please call your doctor's office. It is better to have a medical condition ruled out, and know that the tenderness is a normal part of your development (since you are thirteen, I assume your breasts may still be growing and tender).
Symptoms that you should call your doctor:
- your breast feels warm to the touch (one spot will feel warm or not, and the other skin feels normal)
- any discharge from your nipples
- any pain or itchiness