I've had problems with the milk ducts for many years. They have given me pain and feels like wire behind the nipple and lower part of the breast tissue. Over the years I have had surgery on the ducts as they were dilated and  became infected. 
I hope you have just had a niggle and that the ducts calm down again if you are still worrying and sore l would see the doctor. 
take care 

Susie 

Hello and thanks for your post,

I am sorry to hear about the pain that you have been experiencing in your left breast. It is good to hear that you have had an ultrasound which can back clear.

Breast pain affects many women in one or both breasts and can be linked to different causes such as a change in hormone levels to name one. Breast Cancer Now have produced some useful information about the different types of pain and tips to help relieve it which you can see here

Breast Cancer Now have information on how to examine your breasts and signs to look out for which you can see here

Doctors working in breast clinics are very experienced at looking out  for any warning signs. I am sure that if they were concerned about marks on your breast they would have looked in to it in more detail. Stretch marks are very common and usually present as lines or streaks across the skin. The NHS have some information on what stretch marks look like here 

I do hope that as time goes on the pain may get better for you. If it does continue do go back and discuss this with your GP to see if there is anything that they can suggest to help relieve your symptoms. Try to wear a supportive bra which may help relieve the pain.

I hope this helps.

All the Best,

Catherine