I Have Been Having Discomfort In My Right Side Below

I have been having discomfort in my right side below my rib cage. It feels sore, and there is a fullness almost like a feeling of a growth (like when I had babies) except in my right side only. I have also noticed periodic lower back pain. I have recently experienced fatigue and a decrease in my ability to run at a fast pace or exert myself as I used to do. I noticed that my heart rate was soaring very high, and I would become short of breath when trying to resume my regular speed. I have gone to the cardiologists and had stress tests, cardiac holter for 24 hours and they have found that I have SVT with AFIB. I don't understand why this has just begun and am concerned there may be another underlying problem.