Sounds as though your muscle was tight or you nicked a nerve(but if you hit your sciatic nerve(runs from your a** to your foot in both legs) you'd know... It would hurt to just walk or sit...

I'm leaning toward your muscle tensing up as you inserted! The main thing is to relax. If your standing put as much of your weight on your opposite leg leaving the leg your pinning in limp... As this will relax your muscle...