Gaining a new old friend

The message had been clear. "Meet me at the usual spot. Tell no one /Calvin"
Fourteen year old Jeremy stared down at his phone. A message from his friend Calvin that had been gone for a weeks and now he came back from nowhere and wanted to meet?
He pushed the pedals on his bicycle as quickly as he could.

When Jeremy arrived the spot was already taken. By an college girl no less, and Calvin was nowhere to be seen.
"Oh great", he thought when the girl turned around and began to wave at him. He waved back and was just about to move further away when he heard her shout his name. "Jeremy! Over here!"
He stopped dead in his tracks. Did she know Calvin?

Jeremy headed over to her.
"Here. Sit down", she said and patted the grass next to where she was sitting.
"You probably is wondering what is happening", she began, "Cutting a long story short; I'm Calvin"
Jeremy was speechless and just stared at the crazy woman.
"I can prove it! You have a scar on your left thigh from when we played with knives in the woods. We said you climbed a fence and tripped just so you wouldn't get in trouble with your mom".

"How did you...", he stammered.

"Or the time we played with the matchsticks and accidentaly set the hay field on fire, called the fire department and then was lauded as heroes?"

"There's no way Calvin could have told you that!"

"That's becaue I'm really Calvin", she grinned.

"Really?"

"Yes. Really."

"How did it happen?" The two of them had been talking for a while but Jeremy had avoided the question up until now.
"Dunno really. It's still hazy. I was in the town to buy candy and this girl runs out from the antiques shop at the corner with this glowing stone and collides with me.
I see a flashing light and when I open my eyes again I'm this girl. It's kinds cool, I've got her memories and everything".

"A real girl?"

"Yeah! Watch!"
Calvin bounce up from her prone position and sticks her butt up into Jeremy's face and tug her skintight shorts upwards. "Can you see it?"

"See what?", a perplexed Jeremy utters.

"Exactly! No dick, no balls"
"Just a smooth groin and a big butt. It feels great!"