I've Had Stomach Pain That Has Gradually Got Worse. I

I've had stomach pain that has gradually got worse. I have constant pain in my left upper abdomen. Hurts to press on it. When I stand it feels better, The pain is ALL the time, I am going crazy. It feels constantly tight, like I just did 50 sit ups. The pain doesn't go away unless I stand up. I have also been fatigued and had mono a couple months ago. Any ideas?