What Causes Pain In Lower Abdomen Radiating To The Back?

Hi, I am a 19 year old female with pain in my lower abdomen, URQ (radiating to sides and back) and sternum pain. This has been going on for quite some time, and my doctor said I had mesenteric lymphadenitis. I took the prednisone and zantac as told, and it reduced my symptoms but I still have sharp pains that come and go in the lower portion of my abdomen. He also said I was distended under my left rib cage (which never went away) and still is very sharp and tender pain, especially bending forward. (also, not sure if its related, but when I lay down my sternum hurts and my chest hurts as well, and when I over-do-it its like my heart being stabbed with a knife, but I have VERY low blood pressure with my systolic in the 90s and diastolic being in the 40s when im normally 120s/70s) I also have found that I am constantly run down and my teachers have questioned me looking extremely pale lately. any idea whats causing this?!