Upper right quadrant discomfort under rib cage

I am a 68 year old man who was diagnosed with Prostate Cancer in 2009. The treatment of choice was Proton Therapy which I consider the best treatment out there without any side effects. Recently I started to have a discomfort, dull pulsing sensation, in my upper right quadrant under my rib cage, that comes and goes.  I have been having Bowel movements( normal stools) but I have been experiencing belching and passing gas a lot. I have good appetite but everytime for the last 4 days I feel weak just right before eating.
I had an X-Ray 3 days ago and my doctor found out that I have in the area mentioned above, stool deposits.  He prescribed Miralax which somehow has improved the sensation in the right side of the rib cage.  I do not have nausea nor I have been vomiting, no fever, but I still feel that there is something not right under my rib cage.  The discomfort radiates toward the back on the right side also.  Should I get a MRI or CT scan?  My doctor told me that with Miralax it should get better, but it is not.  Any idea what is going on? Thanks