Bedtime might give some problems.

The rest of the day precedes fairly normal. Or as normal as can be between 2 people that only recently tried to **** each other. Though it did help that we both realized our mistake in it. It was the shared guilt that kept us together.

When it was getting late, I offered to sleep on the cough, she offered to let me sleep with her in her bed.
“Are you sure you want this Gemma?” I asked her.”I mean, considering what has happened earlier today.”
Gemma pondered a moment. “I have an idea.” She says.
Gemma hugged me and gave me a French kiss. She made sure to press her breasts into my chest. Very soon, I had an erection … and then Gemma let go of me.

Once more I changed in my female form. Then Gemma said “Now I don’t have to worry about you possibly breaking your promise any more. And Chet is not around either, so you don’t need to worry either.”
I didn’t much like it, but she was right.
“Just spend the night as a girl.” Gemma said with a grin. “Tomorrow, I’ll help you turn back into a boy.”
It was clear that she was thinking of something naughty.

But then Gemma grabbed for her stomach. She fell to her knees and doubled over. I couldn’t make out whether her screams were of pleasure of pain, but soon her body started to change. Her breasts became flat, her hips thinner, and her voice got rougher. When she stood back up again it was clear Gemma had turned into a guy.
“What the hell? What happened?” He nearly screamed.
“I don’t know.” I said. “One moment you were a girl, and the other you were a guy.”
Gemma unbuttoned his pants and took a look at his crotch. “Oh my god.” He said. “That changed also.”
You weren’t so much interested in her new form, but more into why she changed. This is your curse. The same one that has been inflicting you. It didn’t jump over to her because it’s still effective in you. The proof being in your body currently being female.

Then it hit you.
“The blood.” You say. “You licked up my blood. It must have contained part of the curse and now it transferred to you.”
But as you turn your gaze back at Gemma you notice he is already stroking his erect cock.
“Doe” He tells you. “There’s something wrong. I can’t stop touching myself.”
The second part of the curse. Once you’re changed into the other gender, you become horny. Since it is so natural to you, you didn’t even realize your pussy was wet.
“Please.” Gemma asks. “Help me.”