Account stolen, level downgraded from 25 to 5, please advice

Dear Steemers,

I can't figure how (edit: I figured it out since), but my account was stolen about one week ago. I figured out something happened because suddenly my password did not worked anymore.

Now, that I managed to recover the account I found my level downgraded from 25 to 5. Also, 2 posts repeated which seems to have shady links in my history. The titles of the posts are:

• The world will witness the fall of some digital currencies
• New Glitch in Steemit

Now, I ask for the advice of advanced users:

1. What might have happened since as far as I know my computer is not compromised. How come the hacker got my account?
2. Now that my level is 5, how will this affect me on Steemit?
3. Is there any way to protect myself from this kind of attacks in the future?

Any advice is welcome!