Ive had 6 operations for diskus on my bag. Im using a belt with some stabilce on the back every time i play table tennis
This is why I added the info on the back. It is very possible that this is related to referred pain from the lower back. What you say above makes it seem more likely. Knees are a different animal than groin. If you are getting a sharp pain, that sounds like nerve pain, that sounds like nerve impingement, not like what IVegita described in his post about the knees. What he described sounded like it could be patellar tracking or ligaments not used to holding the weight.
What you are describing could be from tightness in the area. But that is actually less likely than it being from nerve impingement. It is possible that the twisting action is causing impingement. Many of the nerves in the lumbar spin pass through the pelvic structure at two key areas.
The large nerve depicted in the photo is the sciatic nerve. That one is not of so much interest to us. The smaller nerves that are shown passing through the outlet formed by the pubic bones (called the obturator foramen). Those, impinged during the rotational motion of a FH loop could cause that pain you are feeling.
I would play around with trying to loop with legs and turn of hips, but not not twisting in the lumbar spin. If your hips turn, your torso will also turn. But if your hips turn and your lumbar spin twists, your chest will turn more than your hips. Try and have your chest and hips turn together and not to turn chest more than hips. If that causes that pain to stop, you will have your answer.
Can you post footage of you looping. It is also possible you are putting more effort than is useful into your loops. Not sure without seeing though.