Was lifting a heavy box onto another box vertically & to the right. felt momentary left side abdominal pain for 0.5 sec. any chance internal injury?

Unlikely: Most likely an abdominal muscle strain as it occurred with lifting. If you can reproduce the pain by stretching that area or with flexing of those muscles, it is a likely a muscle strain. The one organ on the left side of your abdomen that is a concern for internal injury is your spleen, but that occurs with trauma to the spleen, not with just lifting. If nausea, fever, vomiting, see MD.