I gained 20 pounds in a few months!!!

Haley • 22.
So, I've always had a small frame. Ive been 5'5" and 90lbs since i hit my growth spurt at 15. I have never been able to put on any weight. But I've gained a little over 20 pounds in, I'd say, a total of 3 months. The weight is staying on, and i love it. Its nice to be 22 and actually have some boobs and butt lol, but what could be the cause of this rapid weight gain out of nowhere. I haven't changed my diet, exercise, or daily routine. Not pregnant, not trying to be. Not on any medications. What do you think could be the cause?