Something happened on Hoth

Rewatch Empire. It takes place, roughly, 2 years after A New Hope ( I think). And in that time he's just hanging out with the princess in the rebel alliance, doing side missions we don't know about and trying to set up a base on Hoth. 

It's cold on Hoth.

So... In That moment when Vader tells Luke he's his father, that look like he's about to throw up that Luke gives them, that's NOT cause his dad's Vader. It's cause he knows... KNOWS... Deep down... He knows he's not the only orphan... There IS another... And what they did... (Not saying it was a HR, but like maybe 3rd base) and how this is going to bring up issues later on at the Skywalker family reunion... 

If you want further proof look at Leia's reaction in Jedi when she figures out Luke's her bro. She kind a looks over to the side like "oh. (Half second pause.. Then) OH, shit"