It may be the way you are sleeping
I found out I was pregnant at 5wks. I actually was relaxing at home when I started getting really horrible take my breath away pain on my left side that went from my stomach up to my shoulder and arm. It scared the **** out of me b.c I thought I could be having some sort of heart attack or something. I was rushed to the er and they did blood work and ever and that's how I found out. The Dr said that the pain I was experiencing was gas pain and it is north get throughout pregnancy. It is very uncomfortable and nerve wracking. I am now 26wks with a healthy baby boy! Good luck to you!!!
Its also a sign of an ectopic pregnancy, I would call your doctor to be checked out just in case!
Could be gas pushing up i to your diaphragm maybe...if it persists ask your doc.