Run now as fast as you can!
I had the same exact thing happen to me but I was married. My wife was hot and sexy but after 3 years of marriage she decide to eat a crappy diet and never stopped. My wife went from 5’4” 100 pounds to 150 pounds in two years. Needless to say she in now my ex-wife and fatter now then ever. I learned an expensive lesson, so run while you can! If you are young it will only get worse not better. She might improve for a short while but you have forever to watch her get fat and theirs a very high percentage chance she will fatten up with time.