No cut and dry answer here. But there are some things that disturb me for example:
Who relieved you from your guard shift?
Where was the Sergeant of the Guard?
What about General Order Number 1? I will guard everything within the limits of my post and quit my post only when properly relieved.
In the field usually either the Sergeant of the Guard or you wake up the next person on shift wait for them to get up get dressed an relieve you at your post. Either of these individuals could verify you pulled your shift.
If however you pulled your shift, and went back to your rack there is a problem or if you woke the person up and did not wait for them to get up and get to the guard location. Either way the guard chain would have been broken and the area left unprotected. BIG PROBLEM!
Things to Consider
You could bring up the process that was used and how you applied the process you were given. You might want to type up your response to present to the Commander during the Article 15 session. That way you can include everything you want to say.
You might want to review this post as I believe it will help you be prepared for the Article 15 hearing.