She's messing with you. All she has to do is yank your string and you vibrate for days. If she is pregnant (a big if), there is no proof it is from the sex with you.
This is one of the better reasons to see a good therapist who specializes in anxiety management. (If you have been seeing one, find a better one. You do not have to have "major anxiety" with all the good help that is out there.) You need to learn some tools to desensitize yourself, or you will always be someone's pawn. Don't just wring your hands and say "alas, I have Major Anxiety so of course I take this hard," get your life back and take back your power. People like her will not be able to manipulate you, and you will probably also make better choices about who to hang around with in the first place.
I am not saying she is not pregnant, but I am saying treat everything she says skeptically. If she had gotten pregnant from the sex with you, as you note, she would probably have mentioned it earlier and not through an oblique reference in social media. She has not been sleeping only with you, this might explain the obliqueness of the reference -- the dates might point to someone else. In this situation, all you are obliged to do is to take a legally approved DNA test when the baby is born. You are definitely not required to lose sleep over it mere days after the sex.
You said "I can't figure it out and it's like a dark cloud over my daily life." Why? when she hasn't even come to you and said you have anything to be dark-clouded about? I see no reason to be a drama king over someone who is not all that stable, you don't know the story of what might be going on with her body, the baby is not likely to be from you if she's really pregnant (or she would be on the phone telling you so every day), and for heaven's sake, the world needs your energy. Move on making your life important and finding something fun to do.