What Causes Pain And Swelling In Foot?

I woke up friday morning and the right side of my left foot was so painful I couldn't walk on it, it's all underneath from the ball of my food to my big toe. It's swollen and changed colour slightly. Yesterday the pain eased a little but started flaring up again late yesterday afternoon and I am in agony again this morning. I can't remember banging or catching my toe/foot on anything. What could it be?