Husband is 36. Has moderatly sharp upper left abdominal pain. Occurs when lying down. Had severe GI upset 3 days last week. Related? What could it be?

Probable reflux. Several possibilities. It sounds like it has not been long term. No mention of injury so probably not spleen, onset with lying down indicates a high probability of esophageal reflux, possibly gastritis. Since it seems to be recent onset (3 days) in a 36 year old man and the pain is now severe, he may have helicobacter pylori gastritis and peptic ulcer disease. .H. Pylori is felt to cause ulcers.
Colon gas or stool. This most likely represents constipation or gas. The colon is attached under the rib cage on both sides. When constipated can get left upper quadrant or right upper quadrant pain from gas and stool. The stomach itself is also in the left upper quadrant and inflammation of it (gastritis) can also cause discomfort there. See your physician if persists.