Gained 12lbs and only 9weeks???

So my normal weight is 126-128lbs. I am 5’8 and naturally thin. I am 9 weeks pregnant and have gained 12lbs... yes, TWELVE POUNDS! I haven’t had my first scan yet, having it next week, but I am fascinated as to how it’s possible I’ve gained so much weight so quickly. I still don’t look pregnant because again, I am naturally thin, so could have probably used some weight on me at 5’8, but from what I’ve heard, I should have only gained a few pounds at this point. Any thoughts???