I did some quests in Whitehelm with the Imperials, then ventured to Windhelm and did some quests (killed the wraith, took the oath). I want to be part of the Stormcloaks for good but I remember the Jarl in Whitehelm promoted me, which one am I part of? Did I become part of the Stormcloaks for good when I took the oath?

Whoever you took the Oath to puts you on that side.

Obvious really.

The Jarl in Whiterun isn't the "empire"... he's kind of stuck in the middle. You go to Solitude to join the Empire.

You joined the stormcloaks, then you became Thane of whiterun. They are different things and you can be both.

You're a Stormcloak. Not necessarily for good, as you have one more chance to change sides. And you're also Thane of Whiterun.

Just don`t expect the Jarl of Whiterun to like you much though, especially if he rewarded you and gave you Lydia.

He makes you feel like a traitor.

Ah. I thought Whiterun was the empire. Thanks for clearing this up.

Every hold capital is led by a Jarl. Hold capitals are those nine major cities marked with their coat of arms in the in-game map. Apparently you've met Balgruuf, the Jarl of Whiterun, and you thought he was the Emperor. Understandable. However, The Emperor resides in Castle Dour in Solitude, behind locked doors where you can't enter. Solitude's Jarl is the widow of the late High King, Torygg. High King is the ancient title for the (Nord) leader of Skyrim, and at the moment, Skyrim doesn't have one. Ulfric Stormcloak, the Jarl of Windhelm and the leader of the Stormcloak rebellion, would love to be one, though.

because he's right

Can't betray someone you never swore allegiance too

And Balgruuf calling someone a traitor is a bit rich, really, considering he secretly has the morals of a tunnel rat.