My big toe has gone numb. Been like this for a month now. What's wrong with it?

Before you suggest i visit the doctor, i HAVE made an appointment but it's not for another three weeks (I live in the UK and our NHS isn't the fastest). It's numb at the top joint of my toe - i could stick a knife in it and wouldn't feel anything. I'm 32 by the way, non-smoker. Any ideas what could be wrong with it? I haven't injured it recently to the best of my knowledge, and wear good footwear. Thanks :-)