It shouldn’t be prolactine if you’re already taking caber. Maybe its her. I faced a similar problem with my actual gf. I was doing it, everything fine, then she is about to come, and she makes a lot of pressure on my dick and gets stiff like a rock sculpture.
I can’t even move correctly, everything gets weird and I loose my erect.
But by the time, I was doing it 1-2 times a day, and I was off. Another possibility is internet porn, that could be the cause.