How do you feel otherwise, have you any other symptoms , have you any light headedness when this occurs, you say when you get up, so are your legs okay the rest of the day ?
How long does the weakness last? Is there any numbness or tingling in your legs when you have the weakness? Is it both legs? Is it the WHOLE leg or just part? Is there any specific position you were laying/sitting in prior to this occurring? How long are you sitting or laying before you get up and have the weakness?