This is a big complicated. It's what they call "transferrative". Not exactly "while". It means while you are doing something, you transfer to do a different thing. "he was swimming, then got a cramp". it means while he was swimming, he stopped and transferred to something else, in this case getting a cramp. This grammar is always a bit complicated, probably takes more listening practice to get a hold of it.

-하다가 when i'm doing something