I Have Had A Pain In My Middle Right Back

I have had a pain in my middle right back area for around 3 weeks. To the point the I can't sleep on my right side. when I stand it disapates, but I feel it when any sot of pressure is on my right middle back. The pain feels internal rather than muscle. I was diagnosed with a malignant melanoma 18 years ago and, of course, the first thing that comes to mid is a mass..