Ny way I can cancel the Stormcloak quest(s)

You can not continue with the Stormcloak quest. Attacking (Stormcloak) or defending (Imperial) Whiterun are your only options if you want to do the civil war quests.

You shouldn't worry too much about it, though - the impact of these quests is fairly negligible.

You can also run away like a punk after getting your orders at the camp and go straight to Ulfric. He will be annoyed, but the quests will continue as normal (the battle will be won in your absence).

Are you, perhaps a Stage Four Vampire? Stage Four Vampires get attacked ON SIGHT, regardless of bounty or anything else.

As per the UESP:

Full vampirism is made up of four distinct stages, starting at stage one and progressing to stage four. You advance one stage for every 24 hours you go without feeding on a sleeping NPC. Feeding always returns you to stage one.

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Vampires are hated and feared by the general population and stage four vampires will be attacked on sight. The four major joinable factions will not be hostile toward stage four vampires so long as you join before you reach stage four and are not caught feeding on any of their members. With Dawnguard installed, NPCs will only attack you if you are in Vampire Lord form.