Since the pain is increased with movements, it is certain to be a muscle pain and not in the breasts.
You have likely sprained your intercostal muscles, the rib muscles, during stretching and this is what is causing you the pain.
The recommended treatment for such arm pain is RICE:
Rest: give rest for 2 days.
Warm fomentation: apply warm fomentation to the area, warm a cloth or towel with an iron and apply it. But it should not be too hot.
Compression: Compress the area with a scarf or towel, this will give relief. You can also buy Rib belt/rib support, this is most helpful. It stabilizes the rib cage and minimizes movement, and this helps to reduce pain greatly. They are available at all pharmacies.
Elevation: keep the shoulder and that side of the ribs elevated above the level of the heart, at night when sleeping use a pillow under the shoulde and ribs. Avoid sleeping with the arm raised above the shoulder.
Painkillers: Take OTC painkillers Motrin/Advil one tab 8 hourly for pain relief.
Exercise: this is the most important. Gentle stretching exercises will cause healing. Look up this youtube video for some exercises:
Rehabilitation: you have also to take care that you do not sprain your muscles. You may be engaging in some activity which is adding to the problem, like watching tv or working at the computer in an awkward position, sleeping in an awkward position, hobbies like cycling, gym exercises, etc. Take care not to strain the rib area.
In most cases, this will take care of the pain. However, if it persists, you have to show a doctor for this to rule out other causes.
Hope I have been able to clarify the situation for you and helped in resolving your query.
If you need more help, please let me know so l can continue to assist you.
Thank you and I wish you the best of health - stay safe!
Dr. P. Jyoti