Graduation paper submitted to conference, suddenly demoted to third author?
Unless authors are arranged alphabetically, it would indeed be expected that the largest contributor be the first listed, and from what you have written that would appear to have been you.

Except...

You speak about the submission entirely remotely. It sounds like you were not involved and did not ask to be involved in the preparation of the conference submission.  Moreover, it appears that you did not have any discussion about authorship with your advisor. I still would generally expect an advisor to keep the student as first author, but if the advisor believes the student has effectively vanished and does not care about the publication, it is not surprising if a difference in perspective may have developed.  For example:


Your advisor may consider the revisions to be more significant than you believe.
Since your thesis should be a public, citable document, your advisor may be viewing this paper as a derivative work.


You really need to have a conversation about this with your advisor.  Start from an assumption of good intent---your advisor might just be a self-promoting jerk, but they also might have good reason for the choice they made, of which you just are not aware.

Finally, even if you don't end up as first author, consider this as a lesson for the future: stay involved in the preparation of any paper on which you expect to be first author!