I've said things to my Master before like "it feels like you're jackhammering my spleen!" but I meant them figuratively, not literally. I never meant to convey that I thought he was actually poking any internal organs. I just wanted a way to express that he felt like he was really, really deep and ramming something in there.
My husband/Master is long but not exceptionally so. However, I am one of those women with a relatively short vaginal canal. It is very easy for someone of my husband's length to hit the end of the fornix, which is as far as you can go in the vagina.
When he has been pounding me hard (esp., as has been said, in the legs-over-shoulders position), I'm often left with a severe ache in my abdomen, but it feels more like menstrual cramps than a pain in my stomach.
The pain can only be lessened (or masked?) by stimulation of my clit -- after he has pounded me we usually either masturbate me to orgasm, or I lie on the cone (a powerful vibrator) for a while to make the cramps subside.
I've also noticed I'm more susceptible to this kind of cramping early in the morning than later in the day.