I don't know for sure, but maybe it has something to do with the tightness of your grip when you are masturbating vs how she feels?
How often do you masturbate? Do you use a really tight grip? No lube?
If you are used to a tighter grip and no lube, try loosening your grip and maybe easing up on the frequency.
Maybe it's a visual thing? You like how it looks when you masturbate on her?
Obviously, you'd know the answers to this better than I would. It doesn't sound like a medical issue, though, since you're able to ejaculate.