I have discomfort in my upper chest just behind my breast only while lying down on that side sleeping. it has been there and told my doctor but no tes?

Breast pain: It is likely a sign of muscle strain/spasm. But, there are also other potential causes. If you've done recent heavy lifting/strenuous exercise, had a cold wherein you coughed a lot, etc, then this is very much possible. If you are having any chest pain, shortness of breath, or fever, however, you should see a doctor ASAP. Best wishes.