I was just playing Skyrim and I ran into some weird glitch. Almost immediately after entering Jarl Siddgeir's Longhouse everyone inside, including the Jarl himself, tried to kill me. It's not like I have a bounty on me or anything. I'm playing a good guy, and I just recently arrived in Falkreath. Do anyone know how to fix this?

Have you installed the unofficial skyrim patch !!!!

I would double check about that bounty, or you have killed someone accidentally...!!!! (happens sometimes...)

This kind of thing used to happen to me quite often on the Xbox version of Skyrim. I think it can be caused by unintentionally attacking or hitting an NPC during combat. Attacking the goat or other non-hostile animal in Falkreath has caused this to happen to me too. It seems to happen a lot less frequently since I switched to the PC and starting using the Unofficial Patches though. Like Mark said (above), you might want to use those (if you don't already). Sometimes if you wait for 10 days or more away from Falkreath (or wherever NPCs are attacking you), their aggression with reset to normal when the cells reset.

Did you attacked any domestic animal or people at Falkreath? Or did you own a bard and attack he/she and slipped away?