Left/back pain only when lying down

For the past 3 weeks I've had a deep dull pain that started more in my left side just under the ribs and is now moving to my back left side. This only happens when I lay down to sleep, during the day I am completely fine, I workout and move my body and feel fine. This pain scales from 4 to 8 out of 10 and wakes me from sleep, I have to sit up in bed to help relieve the pain, it stops minutes after standing up.

I went to my GP about this, she did a urine test and examined me but since I have no other symptoms she said she doesn't know what it could be and said I should wait it out or if I wanted I could have I chest xray. I'm going for the xray soon because I need to know what this is but I'm worried if they don't find anything. I can't sleep with this pain and I just want to know what it could be.