Crampy feet
Are you doing this workout on carpet? Many times when you do lunges on a cushy carpet, your feet work so hard to balance during the "landing" of the lunge that they cramp.
Are you wearing shoes? Many times when people weight train in the comfort of their home, they will do the workout without shoes. This, too, strains the feet. Especially if you are in socks since this can cause you to dig in hard with your feet so that you don't slip.
If you do wear shoes, do they offer good support? If they are either worn out, pinch your feet, or provide too much traction, these are things that could add up to your feet taking on more of the load then your working muscles.
As far as form is concerned, be sure that when you land from a lunge or perform reps in your stationary squat position, that you do not curl your toes under. This also leads to foot cramping.
Hope this helps!