Hi, i am of a opinion that it may be caused due to compression of sciatic nerve(a nerve that runs at the back of thigh)
this results in temporary
numbness in back of thigh.
Need not panic as this occurs in most of the person's.
your sitting posture may greatly influence this type of condition.
If i were your treating doctor, then i would advice the following
1) Need not panic.
2) Do not sit in a same posture for long time. Try to change posture once every 10 minutes.
3) Try to add more cushion i.e. pillow, to reduce compressing effect of hard surface.
This would help you to decrease the pain. If the condition persist then consult your general physician, as he may do some nerve study, or may refer you to a physiotherapist.
Hope I have answered your question. If you have any further questions I will be happy to help