What is the cause of a stabbing pain in my big toe?
It would depend on where on the big toe the pain is. Do you have a bunion, Do you have arthritis. Do you live the good life and have a rich diet which could lead to gout? There is so much which could be causing the pain.
I really am not being funny, have you checked there is nothing coming through the bottom of your shoe? I walked round for 2 days with a drawing pin through the sole of one of my shoes. When I put my hand in the shoe there was nothing there, it only hit my foot when the sole was compressed. As I say, took me 2 days to turn the shoe over to find the drawing pin. I am a numpty though and I am not by any means suggesting you are, but it is worth checking.
More information would be useful to maybe someone helping you for example, where is the pain, how often is the pain, is it there all the time, does it happen at any particular time of the day, during a particular movement etc.