I love to explore so I went off the tracks from my Dawnguard vampire hunter quest to investigate that Redwater place.
I tried that skooma and ended up locked in caves.
I cleared the place killing all that was moving and proceded with my quest.
When I showed up at the castle everyone started to attack me... Why?
I'm a werewolf from my main game and I haven't noticed nor can see anything in my active effects window about becoming vampire or having any dissease.

Is this a bug and how to fix it or do I have to start that bloody DLC from the beginning again?

I'd check to make sure you didn't get a bounty somehow.