Abdominal pain in lower right rib

So, I have been having a dull pain under my lower right rib, and sometimes I also feel it on my lower left rib. The pain kind of dissapears when I am very bussy/stressed out, and it comes back when I am resting. I still eat normally, and food doesn't change the pain. My weight has not changed at all. Here is a summary of my medical record: 32 year old man, very healthy all my life. Last year I was diagnosed positive on H.pylori and mild gastritis. I was treated with antibiotics and prilozec. 2 weeks ago I did an upper endoscopy and they confirmed that my gastritis is gone and that I NO LONGER have H.pylori.
  3 months ago I had a slightly elevated Ca19-9 (41 U/mL, the test is normal if less than 37 U/mL). I have repeated the test 5 times, and 3 of them where elevated, and 2 of them were normal (and it happens to be normal after I take vacations away from the USA and away from my girlfriend). Last week I started with abdominal pains, and they tested me for mononucleosis, and it turned positive.
  Other tests: I already did CT scans (liver, pancreas, kidneys and gallbladder are normal), ultrasound (everything normal), a colonoscopy (also resulted normal), barium enema (shows no alterations), blood tests for pancreas and liver functions (all good), HIV(negative) and Hepathitis (negative) .
  Question: Is it posible that my abdominal pain is related to the mononucleosis? Is it posible that is also related to my elevated CA 19-9 (and that is why it is elevated when I am close to my girlfriend, who gave me mono, and it is normal when I am away from her?) Can this be somehow related to stress? Do I still need to be worried about some kind of cancer (even though my doctor already told me not to worry about)? Thanks a lot for any advise.