So, i started doing the dawnguard quest and I had to join the vampire lord thing. so i did the vampire lord main storyline with the bow and whatever.

Then I changed my blood back to werewolf so I could do the fort dawnguard quests.

now, everytime i go to fort dawnguard, EVERYONE attacks me.

I have no bounty anywhere

What do i do?

Um, you joined the OPPOSITE side in the conflict, of COURSE they are going to attack you.

I think if you joined their mortal enemies I think they would attack you on site.

Since the game is working as intended in this case, your only option would be to start a new game if you want to join the other side. You can't do both sides on a given playthrough, it's either one or the other.

Yes, that's the problem with joining the "dark side". If you have a save before you started the Dawnguard DLC load it from there and join the other side. You will have no more problems at the castle then.

How the hell do they KNOW I'm on the other side

You can tell who is a vampire and who is not. My suggestion is find the cure. I stopped being a werewolf with the companions. Plenty of sites on how to cure vampirism.

welcome to video games............

That would not stop them from attacked by the Dawnguard, that is Working as intended and is impossible to join the other side, it is called a consequence.

The developers programmed it that way.

Probably not, but even a 5 septims bounty in markarth could trigger that.

Have you tried to left the area, go back to Skyrim, wait for 24 hours, and the re-enter again ???