I have read that cramping during menstruation is a result of a dilation of the cervix. When my ex-girlfriend got an IUD, she said that it felt like an immense cramping pain as it was lodged inside of her uterus.
Here's a little stretch of that, but may help anyway. I'm thinking that there may be some cervical contact when he uses his finger that could be triggering these cramps. It's possible that his penis simply doesn't have the shape nor the angle during penetration to be causing the same issues.
Try asking him to enter you with his fingers a little shallower and focus on the upward pressure of the g-spot. See if you're still having cramps then.
Hope that helps.
Hello.
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