Why do i have these kind of "mental gaps" when i'm learning, reading, etc? is there a name for this?

I would be studying math and do a very simple and stupid operation like, say, -3+7 and my brain all of a sudden would stop thinking... sometimes is mild, i would quickly go back to reason, but sometimes i freeze completely and would even forget the operation i was doing, and when trying to reasoning again i cannot concentrate.. what's this shit?