As you get up to try the costume on again you see it laying over the chair next to your window.
You get up and head to put it on again and as you reach for it the costume disappears in a puff of smoke.
Wait what. You look around it and wonder what happened. You get dressed quickly and head out to the costume shop. But as you come up to the location it is no longer there. Thinking you are remembering wrong you start to wander around the area thinking it may be down one of the other streets.
As the day goes on you realize it was a one time thing. Magic can only be seen momentarily. But there are some of the feelings you had when you had transformed that remained with you. The strong gruff big monster you were remain in you head and you want to get back to that.
The hunger that was ever present was the first thing to return. You hadn't even realized that while you were wandering around the streets you were picking up snacks all along and as you headed home you could feel the comfortable stretching of your belly. You wanted to continue but you just couldn't fit anything else in. As you found your way home you plopped on the bed with that overly stretched belly sticking straight up in the air. There was no way you could sleep any other way.
As the weeks passed your appetite increased and you grew bigger and bigger being led by a belly that stuck out bigger than any you had ever desired.
But that wasn't the only change. You started wearing leather. A vest that couldn't fit around that expanse that continue to grow. And chaps. You found your desires to frequent gay biker bars increased. Your sexuality wasn't always gay but this became a stronger urge.
A few tatoos and you became that big dominant daddy bear that was the closest you could approach to the fat t-rex. And you were enjoying it.