What Causes Sharp Pulsating Pain On The Left Side Below Rib Cage?

I m a 63 year-old woman, 5 3 , 110 lbs. I eat a whole food, plant-based diet (vegan). For about three weeks, I ve had a localized sharp pulsating pain on my left side just below my rib cage. I can make it subside by sitting in a slightly slouched position or by lying down, especially in a fetal position. But then when I stand up again, it gets really bad. Long walks, however, make the pain subside somewhat. Thinking it was related to GERD (from which I last suffered 3½ years ago, I took a course of Zegerid, but the pain is only getting worse. There don t seem to be any other symptoms, e.g., eating doesn t cause further discomfort. I know I need medical attention, but what kind of doctor should I see? Any guidance you can offer will be greatly appreciated.