why knee pillow cause leg stiffness?

[chueh]

Because I have 2 disc bulging and 1 anterolisthesis, I used a roll-up thin towel under my lumbar and knee pillow under my knees during sleeping.

When I get up to go to the bathroom at night, both legs feel so tired, heavy and stiff.  I go to bathroom quite often even at night, so I shouldn't remain the same position for very long (maybe one sleeping cycle between bathroom breaks).

I'm just wondering why knee pillow help almost everybody else, but not me.  My back does feel more relieved though using the pillows, but what happen to my legs.  It's not the same sensation as sciatica.  

Thanks,