What is this pain and pulsing in my upper left abdomen?

Lots possible... Anatomically, the left upper quadrant (luq) of abdomen includes: spleen, pancreatic tail, stomach, descending duodenum, splenic flexure of the colon, associated vessels. Don't forget the diaphragm, heart, and lower chest are pretty close by. The abdominal musculature, ribs, and skin can all contribute as well to localizing luq pain. An exam is appropriate for clarity of diagnosis and treatment.